src: refactor SSLError case statement

- Don't use both break and return simultaneously.
- Use case:/UNREACHABLE() to enforce that all cases are handled, instead
  of CHECK().

PR-URL: https://github.com/nodejs/node/pull/25861


Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>