lib: posix: mqueue: Do not dereference mqd pointer before null check
lib: posix: mqueue: Do not dereference mqd pointer before null check

The compiler can remove the NULL check since the dereference happens
before it (and assume that the pointer is always valid).

Coverity-Id: 185281

Signed-off-by: Leandro Pereira <51c7437a02b34df685e6b4ecb2e9bca23efe2e1a@intel.com>

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