[clang] [Clang] Prevent null pointer dereference in Sema::​CodeCompleteQualifiedId() (PR #90490)

[Tom Honermann via cfe-commits]

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

The change looks good to me. I recommend giving @sam-mccall a day or so to comment if he wants to.

https://github.com/llvm/llvm-project/pull/90490