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

[Balázs Benics via cfe-commits]

steakhal wrote:

Well, given that this was crashing from clang-14, I'm thinking of not backporting this to clang-23. Touching the solver is a risky business and I don't think worth the risk.
Every vendor can of course choose to cherry-pick into their own release branches if they want to have this earlier.

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