[clang-tools-extra] [clang][dataflow]Use cast_or_null instead cast to prevent crash (PR #68510)

[Yitzhak Mandelbaum via cfe-commits]

https://github.com/ymand requested changes to this pull request.

Thanks for this fix! Unfortunately, I wasn't able to repro the crash in godbolt: https://godbolt.org/z/s741z5djY.  Can you double check that the check crashes on that example without your fix?

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