avoid checking leftover length on unescaping
avoid checking leftover length on unescaping

Ruby always null terminates strings for compatibility with
standard C string functions, so as long as our IS_HEX() macro is
working properly and failing on '\0', there is no need to check
the length of the leftover string.

maisumakun/escape_utils,brianmario/escape_utils,brianmario/escape_utils,maisumakun/escape_utils,brianmario/escape_utils