return empty slice when there are no gcps

[bavardage]

• I agree to follow the project's code of conduct.
• I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

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Noticed this when upgrading to latest rust/running tests

--- STDOUT:              mio-gdal metadata::test::valid_nitf ---

running 1 test

--- STDERR:              mio-gdal metadata::test::valid_nitf ---
thread 'metadata::test::valid_nitf' panicked at library/core/src/panicking.rs:156:5:
unsafe precondition(s) violated: slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX`
stack backtrace:
   0: rust_begin_unwind
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/std/src/panicking.rs:645:5
   1: core::panicking::panic_nounwind_fmt::runtime
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/core/src/panicking.rs:110:18
   2: core::panicking::panic_nounwind_fmt
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/core/src/panicking.rs:123:9
   3: core::panicking::panic_nounwind
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/core/src/panicking.rs:156:5
   4: core::slice::raw::from_raw_parts::precondition_check
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/core/src/intrinsics.rs:2799:21
   5: core::slice::raw::from_raw_parts
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/core/src/slice/raw.rs:98:9
   6: gdal::gcp::<impl gdal::dataset::Dataset>::gcp

I think when there are no GCPs the resulting pointer data ends up as null, and this then panics even though len is correspondingly 0

[lnicola]

Thanks! I wonder if we have similar issues in other places...