Properly enforce that all ChannelMonitorUpdates are ordered

c99d3d7 updated
`ChannelMonitorUpdate::update_id` to continue counting up even
after the channel is closed. It, however, accidentally updated the
`ChannelMonitorUpdate` application logic to skip testing that
`ChannelMonitorUpdate`s are well-ordered after the channel has been
closed (in an attempt to ensure other checks in the same
conditional block were applied).

This fixes that oversight.

Author: TheBlueMatt

lightning/src/chain/channelmonitor.rs

@@ -3150,8 +3150,11 @@ impl<Signer: EcdsaChannelSigner> ChannelMonitorImpl<Signer> {
 					panic!("Attempted to apply post-force-close ChannelMonitorUpdate that wasn't providing a payment preimage");
 				},
 			}
-		} else if self.latest_update_id + 1 != updates.update_id {
-			panic!("Attempted to apply ChannelMonitorUpdates out of order, check the update_id before passing an update to update_monitor!");
+		}
+		if updates.update_id != LEGACY_CLOSED_CHANNEL_UPDATE_ID {
+			if self.latest_update_id + 1 != updates.update_id {
+				panic!("Attempted to apply ChannelMonitorUpdates out of order, check the update_id before passing an update to update_monitor!");
+			}
 		}
 		let mut ret = Ok(());
 		let bounded_fee_estimator = LowerBoundedFeeEstimator::new(&**fee_estimator);