Remove bogus mon_update_id += 1 fulfilling already-fulfilled HTLCs

If we call get_update_fulfill_htlc (in this case via
ChannelManager::claim_funds_internal ->
Channel::get_update_fulfill_htlc_and_commit) and it finds that we
already have a holding-cell pending HTLC claim, it will return no
monitor update but leave latest_monitor_update_id incremented.

If we later go and add a new monitor update we'll panic as the
updates appear to have been applied out-of-order.

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -1200,6 +1200,8 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
 				match pending_update {
 					&HTLCUpdateAwaitingACK::ClaimHTLC { htlc_id, .. } => {
 						if htlc_id_arg == htlc_id {
+							// Make sure we don't leave latest_monitor_update_id incremented here:
+							self.latest_monitor_update_id -= 1;
 							return Ok((None, None));
 						}
 					},