Fixed out of bounds read in lexer.

Fixed out of bounds read in lexer.

The error was caused by assuming that a sequence of zeroes (used for
padding in YYFILL) cannot form a valid lexeme suffix. This is not the
case with strings, as they may contain arbitrary characters. The fix
is to manually loop over string characters in lexer, stopping at each
zero to check if it's the end of input.

Found by american fuzzy lop (thanks to Henri Salo).