Enforce that node_ids are sorted in channel_announcements

We already enforced that nodes can't have a chanel with themselves,
but the spec was updated to require strict ordering at
lightning/bolts#1333 so we enforce this as
well.

Author: TheBlueMatt

lightning/src/routing/gossip.rs

@@ -2011,9 +2011,9 @@ where
 		&self, short_channel_id: u64, capacity_sats: Option<u64>, timestamp: u64,
 		features: ChannelFeatures, node_id_1: NodeId, node_id_2: NodeId,
 	) -> Result<(), LightningError> {
-		if node_id_1 == node_id_2 {
+		if node_id_1 >= node_id_2 {
 			return Err(LightningError {
-				err: "Channel announcement node had a channel with itself".to_owned(),
+				err: "node_ids in channel_announcements must be sorted".to_owned(),
 				action: ErrorAction::IgnoreError,
 			});
 		};
@@ -2123,6 +2123,13 @@ where
 	{
 		let channels = self.channels.read().unwrap();
 
+		if msg.node_id_1 >= msg.node_id_2 {
+			return Err(LightningError {
+				err: "node_ids in channel_announcements must be sorted".to_owned(),
+				action: ErrorAction::IgnoreError,
+			});
+		}
+
 		if let Some(chan) = channels.get(&msg.short_channel_id) {
 			if chan.capacity_sats.is_some() {
 				// If we'd previously looked up the channel on-chain and checked the script