Initialize callback to NULL to avoid exception on calling uninitialized callback

[Bascy]

Not initializing the callback pointer will cause null pointer exceptions if it is not set manually. If not initialized it will (probably) not be 0 and therefore i.e. PubSubClient.cpp, line 405 will try to call a random address

Some constructors actually set the callback to NULL but most are not.
To make sure that callback will always be set to NULL unless overwritten in a specific constructor or by calling setCallBack() it should be initialized on declaration

[knolleary]

Please add a description to explain what this change is and why it is necessary. Sure it looks simple enough, but why is that needed?

[Bascy]

Updated original comment to explain

[hmueller01]

I would rather use setCallback(NULL); in all PubSubClient constructors that do not contain the MQTT_CALLBACK_SIGNATURE parameter. Currently only PubSubClient()does it.

What do you think about changing all these repetitions of

PubSubClient::PubSubClient(Client& client) {
    this->_state = MQTT_DISCONNECTED;
    setClient(client);
    this->stream = NULL;
    this->bufferSize = 0;
    setBufferSize(MQTT_MAX_PACKET_SIZE);
    setKeepAlive(MQTT_KEEPALIVE);
    setSocketTimeout(MQTT_SOCKET_TIMEOUT);
}

to

PubSubClient::PubSubClient(Client& client) {
    PubSubClient(); // default initialize all members
    setClient(client);
}

and so on ...

[Bascy]

I dont think this repo is maintained anymore ... last changes are from 4 years ago

[hmueller01]

There is a similar approach here #746. @knolleary is willing to do some minor changes #1045, not sure if this will make it.