src: simplify `MessagePort` construction code a bit

Using `ASSIGN_OR_RETURN_UNWRAP` would return if the
created `MessagePort` object had no internal fields.
That would be a bug, so switch to a checked conversion instead.

PR-URL: https://github.com/nodejs/node/pull/23036


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>