Miri test failure

[bjorn3]

Found by @evverx in systemd/systemd#19598 (comment):

test dlmalloc::tests::treemap_alloc_max ... error: Undefined Behavior: null pointer is not a valid pointer for this operation
    --> /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mut_ptr.rs:242:18
     |
242  |         unsafe { intrinsics::offset(self, count) as *mut T }
     |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ null pointer is not a valid pointer for this operation
     |
     = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
     = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

     = note: inside `core::ptr::mut_ptr::<impl *mut u8>::offset` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mut_ptr.rs:242:18

[evverx]

I didn't post the full backtrace there:

     Running unittests (target/x86_64-unknown-linux-gnu/debug/deps/dlmalloc-b4f15f60020ff234)

running 2 tests
test dlmalloc::tests::treemap_alloc_max ... error: Undefined Behavior: null pointer is not a valid pointer for this operation
    --> /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mut_ptr.rs:242:18
     |
242  |         unsafe { intrinsics::offset(self, count) as *mut T }
     |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ null pointer is not a valid pointer for this operation
     |
     = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
     = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

     = note: inside `core::ptr::mut_ptr::<impl *mut u8>::offset` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mut_ptr.rs:242:18
note: inside `dlmalloc::Chunk::to_mem` at src/dlmalloc.rs:1726:9
    --> src/dlmalloc.rs:1726:9
     |
1726 |         (me as *mut u8).offset(2 * (mem::size_of::<usize>() as isize))
     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `dlmalloc::Dlmalloc::<sys::System>::top_foot_size` at src/dlmalloc.rs:200:36
    --> src/dlmalloc.rs:200:36
     |
200  |         self.align_offset(unsafe { Chunk::to_mem(ptr::null_mut()) })
     |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `dlmalloc::Dlmalloc::<sys::System>::max_request` at src/dlmalloc.rs:167:43
    --> src/dlmalloc.rs:167:43
     |
167  |             ((!0 - (DEFAULT_GRANULARITY + self.top_foot_size() + self.malloc_alignment()) + 1)
     |                                           ^^^^^^^^^^^^^^^^^^^^
note: inside `dlmalloc::Dlmalloc::<sys::System>::malloc` at src/dlmalloc.rs:295:27
    --> src/dlmalloc.rs:295:27
     |
295  |         } else if size >= self.max_request() {
     |                           ^^^^^^^^^^^^^^^^^^
note: inside `dlmalloc::tests::setup_treemap::<sys::System>` at src/dlmalloc.rs:1791:30
    --> src/dlmalloc.rs:1791:30
     |
1791 |         let large_request1 = a.malloc(large_request_size);
     |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `dlmalloc::tests::treemap_alloc_max` at src/dlmalloc.rs:1816:13
    --> src/dlmalloc.rs:1816:13
     |
1816 |             setup_treemap(&mut a);
     |             ^^^^^^^^^^^^^^^^^^^^^
note: inside closure at src/dlmalloc.rs:1813:5
    --> src/dlmalloc.rs:1813:5
     |
1813 | /     fn treemap_alloc_max() {
1814 | |         let mut a = Dlmalloc::new(System::new());
1815 | |         unsafe {
1816 | |             setup_treemap(&mut a);
...    |
1819 | |         }
1820 | |     }
     | |_____^
     = note: inside `<[closure@src/dlmalloc.rs:1813:5: 1820:6] as core::ops::FnOnce<()>>::call_once - shim` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
     = note: inside `<fn() as core::ops::FnOnce<()>>::call_once - shim(fn())` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
     = note: inside `test::__rust_begin_short_backtrace::<fn()>` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/test/src/lib.rs:577:5
     = note: inside closure at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/test/src/lib.rs:568:30
     = note: inside `<[closure@test::run_test::{closure#2}] as core::ops::FnOnce<()>>::call_once - shim(vtable)` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
     = note: inside `<alloc::boxed::Box<dyn core::ops::FnOnce() + core::marker::Send> as core::ops::FnOnce<()>>::call_once` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1575:9
     = note: inside `<std::panic::AssertUnwindSafe<alloc::boxed::Box<dyn core::ops::FnOnce() + core::marker::Send>> as core::ops::FnOnce<()>>::call_once` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:347:9
     = note: inside `std::panicking::try::do_call::<std::panic::AssertUnwindSafe<alloc::boxed::Box<dyn core::ops::FnOnce() + core::marker::Send>>, ()>` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:401:40
     = note: inside `std::panicking::try::<(), std::panic::AssertUnwindSafe<alloc::boxed::Box<dyn core::ops::FnOnce() + core::marker::Send>>>` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:365:19
     = note: inside `std::panic::catch_unwind::<std::panic::AssertUnwindSafe<alloc::boxed::Box<dyn core::ops::FnOnce() + core::marker::Send>>, ()>` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:434:14
     = note: inside `test::run_test_in_process` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/test/src/lib.rs:600:18
     = note: inside closure at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/test/src/lib.rs:492:39
     = note: inside `test::run_test::run_test_inner` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/test/src/lib.rs:530:13
     = note: inside `test::run_test` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/test/src/lib.rs:564:28
     = note: inside `test::run_tests::<[closure@test::run_tests_console::{closure#2}]>` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/test/src/lib.rs:305:17
     = note: inside `test::run_tests_console` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/test/src/console.rs:290:5
     = note: inside `test::test_main` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/test/src/lib.rs:122:15
     = note: inside `test::test_main_static` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/test/src/lib.rs:141:5
     = note: inside `main`
     = note: inside `<fn() as core::ops::FnOnce<()>>::call_once - shim(fn())` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
     = note: inside `std::sys_common::backtrace::__rust_begin_short_backtrace::<fn(), ()>` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys_common/backtrace.rs:125:18
     = note: inside closure at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:49:18
     = note: inside `core::ops::function::impls::<impl core::ops::FnOnce<()> for &dyn core::ops::Fn() -> i32 + core::marker::Sync + std::panic::RefUnwindSafe>::call_once` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:259:13
     = note: inside `std::panicking::try::do_call::<&dyn core::ops::Fn() -> i32 + core::marker::Sync + std::panic::RefUnwindSafe, i32>` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:401:40
     = note: inside `std::panicking::try::<i32, &dyn core::ops::Fn() -> i32 + core::marker::Sync + std::panic::RefUnwindSafe>` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:365:19
     = note: inside `std::panic::catch_unwind::<&dyn core::ops::Fn() -> i32 + core::marker::Sync + std::panic::RefUnwindSafe, i32>` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:434:14
     = note: inside `std::rt::lang_start_internal` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:34:21
     = note: inside `std::rt::lang_start::<()>` at /home/vagrant/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:48:5
     = note: this error originates in the attribute macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error