inspector: fix process._debugEnd() for inspector

This change ensures that the WebSocket server can be stopped
(and restarted if needed) buy calling process._debugEnd.

PR-URL: https://github.com/nodejs/node/pull/12777
Fixes: https://github.com/nodejs/node/issues/12559


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>