fix unique= when struct field is a nil pointer (#1041)

Fix https://github.com/go-playground/validator/issues/749

Co-authored-by: Yann Salaün <ysalaun@yubik.io>

baked_in.go

@@ -312,10 +312,15 @@ func isUnique(fl FieldLevel) bool {
 		}
 
 		m := reflect.MakeMap(reflect.MapOf(sfTyp, v.Type()))
+		var fieldlen int
 		for i := 0; i < field.Len(); i++ {
-			m.SetMapIndex(reflect.Indirect(reflect.Indirect(field.Index(i)).FieldByName(param)), v)
+			key := reflect.Indirect(reflect.Indirect(field.Index(i)).FieldByName(param))
+			if key.IsValid() {
+				fieldlen++
+				m.SetMapIndex(key, v)
 			}
-		return field.Len() == m.Len()
+		}
+		return fieldlen == m.Len()
 	case reflect.Map:
 		var m reflect.Value
 		if field.Type().Elem().Kind() == reflect.Ptr {

validator_test.go

@@ -10043,6 +10043,7 @@ func TestUniqueValidationStructPtrSlice(t *testing.T) {
 	}{
 		{A: stringPtr("one"), B: stringPtr("two")},
 		{A: stringPtr("one"), B: stringPtr("three")},
+		{},
 	}
 
 	tests := []struct {