message: Fix MySQL fetch memory leak.
diff: @@ -1264,7 +1264,7 @@ static PHP_METHOD(swoole_mysql_coro_statement, fetch)
  sw_zval_ptr_dtor(&fcn);
  ZVAL_UNREF(stmt->result);
  - if (ret == FAILURE || retval == NULL || ZVAL_IS_NULL(retval))
  + if (ret == FAILURE || retval == NULL)
  {
  if (stmt->result)
  {
  @@ -1275,6 +1275,9 @@ static PHP_METHOD(swoole_mysql_coro_statement, fetch)
  }
  else
  {
  + zval _retval = *retval;
  + sw_zval_ptr_dtor(&retval);
  + retval = &_retval;
  if (php_swoole_array_length(stmt->result) == 0)
  {
  sw_zval_free(stmt->result);
  