[PATCH] Fix invalid memory access in llvm-mc if file ends in non-newline whitespace or comma

[colinl at codeaurora.org]

It looks like substr accepts npos as a start and returns the empty string

/// \param Start The index of the starting character in the substring; if
/// the index is npos or greater than the length of the string then the
/// empty substring will be returned.

http://reviews.llvm.org/D6151