[clang] [clang][dataflow]Use dyn_cast_or_null instead cast to prevent crash (PR #68510)

[Gábor Horváth via cfe-commits]

https://github.com/Xazax-hun commented:

The fix looks great to me! On the other hand, we usually try to add a regression test for each of the fixes. Any chance you could get a minimal reproducer from the codebase you are looking at? 

https://github.com/llvm/llvm-project/pull/68510