lnwallet: cancel the spend notification when a channel's closeObserver exits

This commit ensures that when a channel’s closeObserver is signaled to
exit before a channel closure has been detected, then the resources
dedicated to the pending spend notification can be freed up.

lnwallet/channel.go

@@ -709,9 +709,13 @@ func (lc *LightningChannel) closeObserver(channelCloseNtfn *chainntnfs.SpendEven
 			return
 		}
 
-	// Otherwise, we've be signalled to bail out early by the
+	// Otherwise, we've beeen signalled to bail out early by the
 	// caller/maintainer of this channel.
 	case <-lc.quit:
+		// As we're exiting before the spend notification has been
+		// triggered, we'll cancel the notificaiton intent so the
+		// ChainNotiifer can free up the resources.
+		channelCloseNtfn.Cancel()
 		return
 	}