[Preprocessor] Fix a crash when handling non-alpha include header.

Summary: the crash is casued by an assertion in StringRef.
(llvm::StringRef::front() const: Assertion `!empty()' failed.)

Reviewers: sammccall

Subscribers: jsji, cfe-commits

Differential Revision: https://reviews.llvm.org/D52721

llvm-svn: 343481