discovery+gossip: make sure we dont advertise node anns with bad DNS

We may have already persisted node announcements that have multiple DNS
addresses since we may have received them before updating our code to
check for this. So here we just make sure not to send these on to our
peers.

discovery/chan_series.go

@@ -192,6 +192,13 @@ func (c *ChanSeries) UpdatesInHorizon(chain chainhash.Hash,
 			return nil, err
 		}
 
+		if err := netann.ValidateNodeAnnFields(nodeUpdate); err != nil {
+			log.Debugf("Skipping forwarding invalid node "+
+				"announcement %x: %v", nodeAnn.PubKeyBytes, err)
+
+			continue
+		}
+
 		updates = append(updates, nodeUpdate)
 	}
 
@@ -296,8 +303,11 @@ func (c *ChanSeries) FetchChanAnns(chain chainhash.Hash,
 					return nil, err
 				}
 
-				chanAnns = append(chanAnns, nodeAnn)
-				nodePubsSent[nodePub] = struct{}{}
+				err = netann.ValidateNodeAnnFields(nodeAnn)
+				if err == nil {
+					chanAnns = append(chanAnns, nodeAnn)
+					nodePubsSent[nodePub] = struct{}{}
+				}
 			}
 		}
 		if edge2 != nil {
@@ -315,8 +325,11 @@ func (c *ChanSeries) FetchChanAnns(chain chainhash.Hash,
 					return nil, err
 				}
 
-				chanAnns = append(chanAnns, nodeAnn)
-				nodePubsSent[nodePub] = struct{}{}
+				err = netann.ValidateNodeAnnFields(nodeAnn)
+				if err == nil {
+					chanAnns = append(chanAnns, nodeAnn)
+					nodePubsSent[nodePub] = struct{}{}
+				}
 			}
 		}
 	}

discovery/gossiper.go

@@ -2229,7 +2229,12 @@ func (d *AuthenticatedGossiper) fetchNodeAnn(ctx context.Context,
 		return nil, err
 	}
 
-	return node.NodeAnnouncement(true)
+	nodeAnn, err := node.NodeAnnouncement(true)
+	if err != nil {
+		return nil, err
+	}
+
+	return nodeAnn, netann.ValidateNodeAnnFields(nodeAnn)
 }
 
 // isMsgStale determines whether a message retrieved from the backing