Fix for negative step

Author: sg3-141-592

gronx_test.go

@@ -60,6 +60,9 @@ func TestIsValid(t *testing.T) {
 		if !gron.IsValid("* 00 * * *") {
 			t.Errorf("expected true, got false")
 		}
+		if !gron.IsValid("0-0/-005 * * * *") {
+			t.Errorf("expected true, got false")
+		}
 	})
 
 	t.Run("is not valid", func(t *testing.T) {

validator.go

@@ -14,7 +14,7 @@ func inStep(val int, s string, bounds []int) (bool, error) {
 	if err != nil {
 		return false, err
 	}
-	if step == 0 {
+	if step <= 0 {
 		return false, errors.New("step can't be 0")
 	}