Fix option argument check that is not executed anyway

cmd/restic/cmd_forget.go

@@ -136,10 +136,11 @@ func init() {
 }
 
 func verifyForgetOptions(opts *ForgetOptions) error {
-	if opts.Last < -1 || opts.Hourly < -1 || opts.Daily < -1 || opts.Weekly < -1 ||
-		opts.Monthly < -1 || opts.Yearly < -1 {
-		// TODO: this seem to have no effect. there is a different error message showing up, e.g. for --keep-last -5
-		return errors.Fatal("negative values other than -1 are not allowed for --keep-*")
+	// Negative numbers for these options are not allowed.
+	if opts.Last < 0 || opts.Hourly < 0 || opts.Daily < 0 || opts.Weekly < 0 ||
+		opts.Monthly < 0 || opts.Yearly < 0 {
+		// This seem to have no effect though. The condition is already catched in `Set(s string)`, line 44.
+		return errors.Fatal("negative values are not allowed for --keep-*")
 	}
 
 	var allKeepWithinXUndefined = true