cluster: close ownerless handles on disconnect()

When a worker is disconnecting, it shuts down all of the handles
it is waiting on. It is possible that a handle does not have an
owner, which causes a crash. This commit closes such handles
without accessing the missing owner.

Fixes: https://github.com/nodejs/node/issues/6561
PR-URL: https://github.com/nodejs/node/pull/6909


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>