Async sample callback context fix.

[CIPop]

Fixing MQTTAsync sample callback context: within the callback, the context is casted to the client handle but configured as 'NULL'. This causes a crash when the connection is lost.

Repro steps

• Set a breakpoint in onConnect

• In another terminal:

netstat | grep 1883
tcp        0      0 mydevstation:32132 137.135.83.217:1883     ESTABLISHED

sudo ss -K dst 137.135.83.217 dport = 1883

• Resume execution after the connection is dropped

Expected: sample reconnects (connlost is called).
Actual: crash in MQTTAsync_connect as client is NULL.