fixed issues found by go-fuzz

helper.go

@@ -12,6 +12,15 @@ const (
 	anys = string(any)
 )
 
+func checkFields(s []string, val string) error {
+	for _, v := range s {
+		if v == "" {
+			return errors.New("invalid subfield: " + val)
+		}
+	}
+	return nil
+}
+
 type less func(i, j int) bool
 
 func reverse(f less) less {
@@ -20,12 +29,18 @@ func reverse(f less) less {
 	}
 }
 
-func index(f Filter, i int, fields []string) (val reflect.Value) {
-	val = reflect.ValueOf(f.Index(i)).FieldByName(fields[0])
-	for j := 1; j < len(fields); j++ {
-		val = val.FieldByName(fields[j])
+func walkFields(v reflect.Value, fields []string) reflect.Value {
+	for i := 0; i < len(fields); i++ {
+		if v.Kind() != reflect.Struct {
+			return reflect.ValueOf(nil)
+		}
+		v = v.FieldByName(fields[i])
 	}
-	return
+	return v
+}
+
+func index(f Filter, i int, fields []string) (val reflect.Value) {
+	return walkFields(reflect.ValueOf(f.Index(i)).FieldByName(fields[0]), fields[1:])
 }
 
 func makeLess(f Filter, fields []string) less {