Fix panic if miss notification sock is nil

Signed-off-by: Jana Radhakrishnan <mrjana@docker.com>

libnetwork/drivers/overlay/ov_network.go

@@ -510,7 +510,10 @@ func (n *network) initSandbox() error {
 		}
 	})
 
-	go n.watchMiss(nlSock)
+	if nlSock != nil {
+		go n.watchMiss(nlSock)
+	}
+
 	return nil
 }