Fix buffer overflow in Lexer

Summary:
Fix PR22407, where the Lexer overflows the buffer when parsing
 #include<\
(end of file after slash)

Test Plan:
Added a test that will trigger in asan build.
This case is also covered by the clang-fuzzer bot.

Reviewers: rnk

Reviewed By: rnk

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D9489

llvm-svn: 236466