message: free memory when the stmt object is destroyed.
diff: @@ -1007,6 +1007,8 @@ static PHP_METHOD(swoole_mysql_coro_statement, __destruct)
  }
  efree(stmt->object);
  stmt->object = NULL;
  + swLinkedList_remove(stmt->client->statement_list, stmt);
  + efree(stmt);
  }
  #ifdef SW_USE_MYSQLND
  @@ -1074,7 +1076,8 @@ static PHP_METHOD(swoole_mysql_coro, __destruct)
  {
  swoole_mysql_coro_close(getThis());
  }
  - if (client->buffer) {
  + if (client->buffer)
  + {
  swString_free(client->buffer);
  }
  efree(client);
  