character code in hexadecimal representation (ASCII)
destination buffer for the UTF-8 sequence of the character; the length must be at least 6; may contain tailing junk if the sequence is actually shorter
the UTF-8 sequence (slices the valid data in utf8_buf) on success or an empty string on failure.
In: - hex.length must be 4, - utf8_buf.length must at least be 6.
Out: The returned string slices utf8_buf from the beginning.
Converts hex, which is expected to contain a 4-digit ASCII hexadecimal number, into its corresponding UTF-8 character sequence.