* Folding results should not be empty.

  If `OnigCodePointCount(to->n)` were 0, `for` loop using `fn`
  wouldn't execute and `ncs` elements are not initialized.

  ```
  enc/unicode.c:557:21: warning: 'ncs[0]' may be used uninitialized in this function [-Wmaybe-uninitialized]
    557 |  for (i = 0; i < ncs[0]; i++) {
        |                  ~~~^~~
  ```

* Cast to `enum yytokentype`

  Additional enums for scanner events by ripper are not included
  in `yytokentype`.

  ```
  ripper.y:7274:28: warning: implicit conversion from 'enum <anonymous>' to 'enum yytokentype' [-Wenum-conversion]
  ```