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: nodejs#6561
PR-URL: nodejs#6909
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

lib/cluster.js

@@ -719,7 +719,11 @@ function workerInit() {
       const handle = handles[key];
       delete handles[key];
       waitingCount++;
-      handle.owner.close(checkWaitingCount);
+
+      if (handle.owner)
+        handle.owner.close(checkWaitingCount);
+      else
+        handle.close(checkWaitingCount);
     }
 
     checkWaitingCount();