cmd/loop: fix channel parsing

cmd/loop/loopout.go

@@ -89,16 +89,21 @@ func loopOut(ctx *cli.Context) error {
 		return err
 	}
 
-	// Parse outgoing channel set.
+	// Parse outgoing channel set. Don't string split if the flag is empty.
-	chanStrings := strings.Split(ctx.String("channel"), ",")
+	// Otherwise strings.Split returns a slice of length one with an empty
+	// element.
 	var outgoingChanSet []uint64
+	if ctx.IsSet("channel") {
+		chanStrings := strings.Split(ctx.String("channel"), ",")
 		for _, chanString := range chanStrings {
 			chanID, err := strconv.ParseUint(chanString, 10, 64)
 			if err != nil {
-			return err
+				return fmt.Errorf("error parsing channel id "+
+					"\"%v\"", chanString)
 			}
 			outgoingChanSet = append(outgoingChanSet, chanID)
 		}
+	}
 
 	var destAddr string
 	switch {