This can happen as we look for '<<<<' while scanning tokens but then expect
'<<<<\n' to tell apart perforce from diff3 conflict markers. Just harden
the pointer arithmetic.

Found by libfuzzer + asan!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@265125 91177308-0d34-0410-b5e6-96231b3b80d8