Find the rightmost occurrence of the given Unicode character
in a UTF-8 encoded string, while limiting the search to @len bytes.
If @len is -1, allow unbounded search.
Return: %NULL if the string does not contain the character,
otherwise, a pointer to the start of the rightmost occurrence
of the character in the string.
Find the rightmost occurrence of the given Unicode character in a UTF-8 encoded string, while limiting the search to @len bytes. If @len is -1, allow unbounded search.