Now the function utf8_next_char with an empty string returns NULL instead of
the next char, which is most of the time after an allocated buffer.
And the function utf8_char_size with an empty string now returns 0 instead of
1.
This indirectly fixes a buffer overflow in function eval_string_range_chars
when the input string is empty (for example when doing `/eval -n ${chars:}`).
Sébastien Helleu
d475c16671