Relax OnionMessenger::peer_disconnected assertion

When a peer is connected, OnionMessenger tracks it only if it supports
onion messages. On disconnect, we debug_assert that the peer was in a
state ConnectedPeer, failing when it is in the PendingConnection state.
However, we were mistakenly asserting for peers that we were not
tracking (i.e., that don't support onion messages). Relax the check to
not fail on the latter.

lightning/src/onion_message/messenger.rs

@@ -914,7 +914,8 @@ where
 	fn peer_disconnected(&self, their_node_id: &PublicKey) {
 		match self.message_recipients.lock().unwrap().remove(their_node_id) {
 			Some(OnionMessageRecipient::ConnectedPeer(..)) => {},
-			_ => debug_assert!(false),
+			Some(_) => debug_assert!(false),
+			None => {},
 		}
 	}