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

[Balázs Benics via cfe-commits]

https://github.com/steakhal approved this pull request.

LGTM with final nits.
Please update the PR title and summary to match what the patch represents now.

We can also keep a pthreadlock checker test if you want.

@NagyDonat Could you test this too? My plan is to land this, and backport it to clang-23 as this would fix a crash regression.

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