worker: fix crash when .unref() is called during exit

To be more precise, fix a crash when `worker.unref()` is called
from a message on the Worker that is not emitted before the Worker
thread has stopped.

PR-URL: https://github.com/nodejs/node/pull/33394


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>