Fix utf16At string binding function.

For most of the utf16At calls we are passing in an index of offset of 0,
i.e. the utf8 character is at the very beginning of the string. The old
way would call webvtt_skip_utf8 which views an offset of 0 as invalid
because it has nothing to skip. Therefore, utf8_to_utf16 would
never be called. I've updated the condition so that it will call the
conversion function if offset == 0.