[lex] Don't read past the end of the buffer

While dereferencing ThisTokEnd is fine and we know that it's not in
[a-zA-Z0-9_.], ThisTokEnd[1] is really past the end.

Found by asan and with a little help from clang-fuzz.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@233491 91177308-0d34-0410-b5e6-96231b3b80d8