Add is_utf8_char_buf()

Add is_utf8_char_buf()

This function is to replace is_utf8_char(), and requires an extra
parameter to ensure that it doesn't read beyond the end of the buffer.

Convert is_utf8_char() and the only place in the Perl core to use the
new one, assuming in each that there is enough space.

Thanks to Jarkko Hietaniemi for suggesting this function name