Fix buffer overrun in SimpleParser::handleBackslash

Summary:
It read 4 chars, then checked for validity, but any of them could have
been the end of the string, so check after each one instead.

Reviewed By: oulgen

Differential Revision: D19611163

fbshipit-source-id: 3da0a39555cb85a93f4fd98048368f17cf37e2e4