Fully initialise the oob struct in nxt_socket_msg_oob_init()

valgrind(1) was producing the following alert

  ==166470== Syscall param sendmsg(msg.msg_control) points to uninitialised byte(s)
  ==166470==    at 0x4AE6514: sendmsg (sendmsg.c:28)
  ==166470==    by 0x42D86C: nxt_sendmsg (nxt_socket_msg.c:32)
  ==166470==    by 0x4FE6695: nxt_unit_sendmsg (nxt_unit.c:6013)
  ==166470==    by 0x4FEB6E2: nxt_unit_ready (nxt_unit.c:963)
  ==166470==    by 0x4FEB6E2: nxt_unit_init (nxt_unit.c:557)
  ==166470==    by 0x4FEEC56: nxt_php_start (nxt_php_sapi.c:507)
  ==166470==    by 0x426BA0: nxt_app_setup (nxt_application.c:1029)
  ==166470==    by 0x403153: nxt_process_do_start (nxt_process.c:718)
  ==166470==    by 0x4042A3: nxt_process_whoami_ok (nxt_process.c:846)
  ==166470==    by 0x407A28: nxt_port_rpc_handler (nxt_port_rpc.c:347)
  ==166470==    by 0x407E42: nxt_port_handler (nxt_port.c:184)
  ==166470==    by 0x40501B: nxt_port_read_msg_process (nxt_port_socket.c:1271)
  ==166470==    by 0x4055B3: nxt_port_read_handler (nxt_port_socket.c:778)
  ==166470==  Address 0x1ffefffc9c is on thread 1's stack
  ==166470==  in frame #3, created by nxt_unit_init (nxt_unit.c:428)
  ==166470==  Uninitialised value was created by a stack allocation
  ==166470==    at 0x4FEABFE: nxt_unit_init (nxt_unit.c:436)

This was due to the nxt_send_oob_t oob structure not being fully
initialised.

Given the name and intention of this function lets *fully*
empty-initialise this structure.

Link: <https://en.cppreference.com/w/c/language/initialization#Empty_initialization>
Signed-off-by: Andrew Clayton <[email protected]>

Author: ac000

src/nxt_socket_msg.h

@@ -89,6 +89,8 @@ nxt_socket_msg_oob_init(nxt_send_oob_t *oob, int *fds)
     int             nfds;
     struct cmsghdr  *cmsg;
 
+    *oob = (nxt_send_oob_t) {};
+
 #if (NXT_HAVE_MSGHDR_CMSGCRED)
     cmsg = (struct cmsghdr *) (oob->buf);
     /*