fix(mbyte): fix bugs in utf_cp_*_off() functions

Problems:
- Illegal bytes after valid UTF-8 char cause utf_cp_*_off() to fail.
- When stream isn't NUL-terminated, utf_cp_*_off() may go over the end.
Solution: Don't go over end of the char of end of the string.

src/nvim/buffer.c

@@ -3436,7 +3436,7 @@ void maketitle(void)
       int len = (int)strlen(buf_p);
       if (len > 100) {
         len -= 100;
-        len += utf_cp_tail_off(buf_p, buf_p + len) + 1;
+        len += utf_cp_bounds(buf_p, buf_p + len).end_off;
         buf_p += len;
       }
       STRCPY(icon_str, buf_p);