Make sure broadcast queue is valid broadcasting

When broadcasting table event, make sure the broadcast queue is
valid. The network may have been removed while in the process of sending
the broadcast.

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

libnetwork/networkdb/broadcast.go

@@ -80,10 +80,26 @@ func (nDB *NetworkDB) sendTableEvent(event TableEvent_Type, nid string, tname st
 		return err
 	}
 
+	var broadcastQ *memberlist.TransmitLimitedQueue
 	nDB.RLock()
-	broadcastQ := nDB.networks[nDB.config.NodeName][nid].tableBroadcasts
+	thisNodeNetworks, ok := nDB.networks[nDB.config.NodeName]
+	if ok {
+		// The network may have been removed
+		network, networkOk := thisNodeNetworks[nid]
+		if !networkOk {
+			nDB.RUnlock()
+			return nil
+		}
+
+		broadcastQ = network.tableBroadcasts
+	}
 	nDB.RUnlock()
 
+	// The network may have been removed
+	if broadcastQ == nil {
+		return nil
+	}
+
 	broadcastQ.QueueBroadcast(&tableEventMessage{
 		msg:   raw,
 		id:    nid,