Avoid parsing PublicKeys when applying an unsigned chan update

`PublicKey` parsing is relatively expensive as we have to check if
the point is actually on the curve. To avoid it, our `NetworkGraph`
uses `NodeId`s which don't have the validity requirement.

Sadly, we were always parsing the broadcasting node's `PublicKey`
from the `node_id` in the network graph whenever we see an update
for that channel, whether we have a corresponding signature or not.

Here we fix this, only parsing the public key (and hashing the
message) if we're going to check a signature.

Author: TheBlueMatt

lightning/src/routing/gossip.rs

@@ -2537,7 +2537,7 @@ where
 				}
 			};
 
-		let node_pubkey;
+		let mut node_pubkey = None;
 		{
 			let channels = self.channels.read().unwrap();
 			match channels.get(&msg.short_channel_id) {
@@ -2556,17 +2556,26 @@ where
 					} else {
 						channel.node_one.as_slice()
 					};
-					node_pubkey = PublicKey::from_slice(node_id).map_err(|_| LightningError {
-						err: "Couldn't parse source node pubkey".to_owned(),
-						action: ErrorAction::IgnoreAndLog(Level::Debug),
-					})?;
+					if sig.is_some() {
+						node_pubkey = Some(PublicKey::from_slice(node_id).map_err(|_| LightningError {
+							err: "Couldn't parse source node pubkey".to_owned(),
+							action: ErrorAction::IgnoreAndLog(Level::Debug),
+						})?);
+					}
 				},
 			}
 		}
 
-		let msg_hash = hash_to_message!(&message_sha256d_hash(&msg)[..]);
 		if let Some(sig) = sig {
-			secp_verify_sig!(self.secp_ctx, &msg_hash, &sig, &node_pubkey, "channel_update");
+			let msg_hash = hash_to_message!(&message_sha256d_hash(&msg)[..]);
+			if node_pubkey.is_none() {
+				debug_assert!(false);
+				let err = "Reached unreachable code".to_owned();
+				return Err(LightningError {
+					err, action: ErrorAction::IgnoreAndLog(Level::Error),
+				});
+			}
+			secp_verify_sig!(self.secp_ctx, &msg_hash, &sig, &node_pubkey.unwrap(), "channel_update");
 		}
 
 		if only_verify {