core: fix buffer overflow in function utf8_next_char and return NULL for empty string

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:}`).

src/gui/gui-key.c

@@ -1328,7 +1328,7 @@ gui_key_chunk_seems_valid (const char *chunk)
     if (!found)
         chunk = utf8_next_char (chunk);
 
-    if (chunk[0])
+    if (chunk && chunk[0])
         return 0;
 
     return 1;