[clang] [Clang][NFC] Fix potential null dereference in encodeTypeForFunctionPointerAuth (PR #104737)

[Tom Honermann via cfe-commits]

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

Thanks, @smanna12! The change looks clearly correct since the type has already been verified.

https://github.com/llvm/llvm-project/pull/104737