Fix: panic: reflect: slice index out of range (#1066)

* Fix: panic: reflect: slice index out of range

* Update interpreter.go

* [no ci] Return null for negative indexes

* Add tests for index access

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

pkg/exprparser/interpreter.go

@@ -170,6 +170,9 @@ func (impl *interperterImpl) evaluateIndexAccess(indexAccessNode *actionlint.Ind
 	case reflect.Int:
 		switch leftValue.Kind() {
 		case reflect.Slice:
+			if rightValue.Int() < 0 || rightValue.Int() >= int64(leftValue.Len()) {
+				return nil, nil
+			}
 			return leftValue.Index(int(rightValue.Int())).Interface(), nil
 		default:
 			return nil, fmt.Errorf("Unable to index on non-slice value: %s", leftValue.Kind())