d/libnetwork: fix (*Controller).getLBIndex panics

Check the serviceBinding's deleted flag to guard against the service
getting deleted before we locked its mutex, which would make it futile
to look up the load balancers. And guard against dereferencing a nil
*loadBalancer in general.

Signed-off-by: Cory Snider <csnider@mirantis.com>

daemon/libnetwork/service_common.go

@@ -160,9 +160,14 @@ func (c *Controller) getLBIndex(sid, nid string, ingressPorts []*PortConfig) int
 	}
 
 	s.Lock()
+	defer s.Unlock()
+	if s.deleted {
+		return 0
+	}
 	lb := s.loadBalancers[nid]
-	s.Unlock()
-
+	if lb == nil {
+		return 0
+	}
 	return int(lb.fwMark)
 }