[clang] [clang][dataflow] Reorder checks to protect against a null pointer dereference. (PR #66764)

[Fangrui Song via cfe-commits]

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

Thanks. It'll be nice to have a test, but I think it is fine to land this now... and will address an internal crash.

https://github.com/llvm/llvm-project/pull/66764