message: Fix columns memory leak.
diff: @@ -353,6 +353,7 @@ static void mysql_columns_free(mysql_client *client)
  }
  }
  efree(client->response.columns);
  + client->response.columns = NULL;
  }
  #ifdef SW_MYSQL_DEBUG
  @@ -1939,7 +1940,10 @@ int mysql_response(mysql_client *client)
  n_buf -= ret;
  buffer->offset += (5 + ret);
  client->response.num_column = client->statement->field_count;
  + if (client->response.num_column > 0)
  + {
  client->response.columns = ecalloc(client->response.num_column, sizeof(mysql_field));
  + }
  if (client->statement->param_count > 0)
  {
  client->state = SW_MYSQL_STATE_READ_PARAM;
  