chainmaker after pointer free should set NULL

[Cavalierliu]

invoke_response->gas_used = transactation_info->transaction->result->contract_result->gas_used;
if (tx_response != NULL)
{
common__tx_response__free_unpacked(tx_response, NULL);
}

1. in the code above，after tx_response free，should Manually set to NULL

[Cavalierliu]

problem fixed