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

[Arseniy Zaostrovnykh via cfe-commits]

necto wrote:

@steakhal 

> > I tried on godbolt clang 20, 21, 22, trunk with assertions and it didn't crash on the added test cases: https://godbolt.org/z/fhca9Yx7Y
> > What release of clang are impacted? What introduced the alleged crashes?
> 
> This is odd.
> 
> It does reproduce in the today's HEAD, as you can see from the CI run: https://github.com/llvm/llvm-project/actions/runs/29832161989/job/88639329677?pr=210912 (I pushed only the test case without the fix)
> 
> I observed it on Clang 20.1.8 (both built from source and distributed by Guix). Also with `Clang 20.1.2 shipped in the Ubuntu APT. It crashes even with assertions disabled. But I can't reproduce it on Compiler Explorer. I'll try to dig deeper.

The issue on Compiler Explorer was the language: C++ and not C. When compiling it as C file it does crash: https://godbolt.org/z/zGsMch5c9

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