lnwire: validate that gossip messages contain valid TLV

In this commit, we check that the extra bytes appended to gossip
messages contain valid TLV streams. We do this here for:
- channel_announcement
- channel_announcement_2
- channel_update
- channel_update_2
- node_announcement

This is in preparation for the SQL version of the graph store which will
normalise TLV streams at persistence time.

lnwire/channel_update_2.go

@@ -154,7 +154,7 @@ func (c *ChannelUpdate2) DecodeTLVRecords(r io.Reader) error {
 		c.ExtraOpaqueData = tlvRecords
 	}
 
-	return nil
+	return c.ExtraOpaqueData.ValidateTLV()
 }
 
 // Encode serializes the target ChannelUpdate2 into the passed io.Writer