Do not attempt serf query when not initialized

Sometimes, the vxlan kernel code may generate miss
notifications for vxlan bound packets when serf is
not initliazed. In such cases we should not try
doing a query as it will create a panic. We should
error out which will generate a log message.

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

libnetwork/drivers/overlay/ov_serf.go

@@ -151,6 +151,10 @@ func (d *driver) processQuery(q *serf.Query) {
 }
 
 func (d *driver) resolvePeer(nid string, peerIP net.IP) (net.HardwareAddr, net.IPMask, net.IP, error) {
+	if d.serfInstance == nil {
+		return nil, nil, nil, fmt.Errorf("could not resolve peer: serf instance not initialized")
+	}
+
 	qPayload := fmt.Sprintf("%s %s", string(nid), peerIP.String())
 	resp, err := d.serfInstance.Query("peerlookup", []byte(qPayload), nil)
 	if err != nil {