[clang] [NFC][Clang][Lex] Add assert to prevent null pointer dereference in Preprocessor::HandleModuleContextualKeyword (PR #211117)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin approved this pull request.

There are 60+ uses of CurPPLexer - and very few are gated by either an if or an assert - so i hope we won't have to generalize that PR too much.

That being said, I'm not opposed to us landing that PR

https://github.com/llvm/llvm-project/pull/211117