In the mqtt5 protocol the broker can disconnect the client with
a disconnect packet. This packet contains a reason value that can be
useful for certain applications in which it is important to know
the reason of disconnection.
While the client is connected is possible that a disconnect packet
is reaceived by the broker to force a disconnection. Before this
patch this approach causes a generic error on transport in case of
disconnection from the broker. If the packet is managed before getting
an error it is possible to save the reason code in the
disconnect_return_code variable in the error_handle, and dispatch
the disconnect event that can be managed by the application event
loop, that now can know the reason of disconnection from the broker.
Reset the variable in case of error.
Signed-off-by: Flavia Caforio <flavia.caforio@amarulasolutions.com>