fix: client crashes a program after destructor is called

[Kirkezz]

I am using your library as follows:

int getPlayersCount() {
    static int id = 3;
    int result = -1;

    rconpp::rcon_client client(rcon_credentials.ip, rcon_credentials.port, rcon_credentials.password);
    client.on_log = [](const std::string_view& log) { std::cout << log << std::endl; };
    client.start(true);
    if (client.connected) {
        auto response = client.send_data_sync("list", ++id, rconpp::data_type::SERVERDATA_EXECCOMMAND);
        /* ... */
        std::cout << response.data << std::endl;
    }

    return result;
}

int main() {
    ABMinecraft minecraft(Settings::rcon_credentials);
    minecraft.getPlayersCount();
    std::cout << "exited getPlayersCount(), rcon_client is destructed" << std::endl;
    while (true) {
    }
}

The program crashes a moment after the rcon_client is destructed:
I think the problem is that queue_runner uses this->connected after the rcon_client object is destructed, and even if bool connected was set to false in the destructor, it may not be false after the memory is freed.

For a possible fix, see PR.

[Jaskowicz1]

Thank you for your contribution!