[clang] Fix Unintialised char *CommentLexer resulting in potential UB. (PR #210067)

[Aaron Ballman via cfe-commits]

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

LGTM with the minor change applied, presuming precommit CI doesn't find any surprises

https://github.com/llvm/llvm-project/pull/210067