[Lex] Avoid out-of-bounds dereference in SkipLineComment

Credit to OSS-Fuzz for discovery:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=3145

rdar://34526482

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@315785 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 4318ef1cb398b9b72aea287a815c09cfe68a2b27)
2 files changed