Finds the previous UTF-8 character in the string before
p does not have to be at the beginning of a UTF-8 character. No check
is made to see if the character found is actually valid other than
it starts with an appropriate byte. If
p might be the first
character of the string, you must use
A pointer to a position within a UTF-8 encoded string.
The data is owned by the caller of the function. The value is a NUL terminated UTF-8 string.