[clang] [clang] Fix a segfault when M is a nullptr (PR #130712)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

This needs a release note and it would be helpful to know how you found this and a test w/ a minimal reproducer. Are there any bugs reports for this?

https://github.com/llvm/llvm-project/pull/130712