fix race with defaultTableName field (again)

model_struct.go

@@ -24,11 +24,16 @@ type ModelStruct struct {
 	PrimaryFields    []*StructField
 	StructFields     []*StructField
 	ModelType        reflect.Type
+
 	defaultTableName string
+	l sync.Mutex
 }
 
 // TableName returns model's table name
 func (s *ModelStruct) TableName(db *DB) string {
+	s.l.Lock()
+	defer s.l.Unlock()
+
 	if s.defaultTableName == "" && db != nil && s.ModelType != nil {
 		// Set default table name
 		if tabler, ok := reflect.New(s.ModelType).Interface().(tabler); ok {