Reject NULL / uninitialised mutex handles with EINVAL - #3

Summary

• Make pthread_mutex_{lock,trylock,timedlock,unlock,destroy} reject mutex == NULL and *mutex == NULL with EINVAL instead of dereferencing.
• The previous code relied on the OS to convert a bad load into an error. On the embedded targets this library runs on (PS2 EE, PSP, Vita) there is no signal handling for bad loads — the kernel traps and the process dies, so the API itself has to validate the pointer.
• Add tests/mutex9.c exercising both NULL handles and the post-failed-init NULL-value case across all five mutex APIs.

Test plan

• Build the test suite for PSP and confirm pthread_test_mutex9() passes.
• Build the test suite for PS2 and confirm pthread_test_mutex9() passes.
• Confirm existing mutex tests (1..8r) still pass on both targets.