[clang] [analyzer] Fix null-pointer dereference in PthreadLockChecker (PR #210912)

[Arseniy Zaostrovnykh via cfe-commits]

necto wrote:

> You should consider my comments across these hunks and while the comments were useful for understanding and verifying the claims, I think we could drop 80% of them to keep the most pressing 20% that actually matters.

I believe I addressed your comments, and I trimmed majority of my comments:
57b0790fdd4e [NFC] Trim extra comments

> I'd like to see some empirical evidence that this wouldn't cause FPs. I know it's probably a lot more work than the originally proposed patch, so I understand that this turned out to be a time sink for you, but this is a core component and we must get this right.

I did run analysis on 200+ opensource projects and it revealed no issue difference beyond the regular noise levels or any performance effect beyond the CI noise.

https://github.com/llvm/llvm-project/pull/210912