The asynchronous 'SetCallbacks' function requires a 'MessageArrived' function even in a publication scenario

[senajoaop]

The documentation page for the Paho Asynchronous MQTT C Client Library contains an example of a publication, which have:

[...]
MQTTAsync_setCallbacks(client, NULL, connlost, NULL, NULL);
[...]

This example doesn't work based on the if verification in line 1521 in the MQTTAsync.c file, which checks if the MQTTAsync_messageArrived* pointer to a callback functions is NULL.