Use correct minimum value for PacketIdentifiers

[AeroStun]

The MQTT spec bans the PacketIdentifier 0 on PUBLISH packets; non-permissive clients such as mosquitto_sub die whenever they recieve a QoS > 0 message. Same goes for the Rust library Rumqttc.

See http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718025

SUBSCRIBE, UNSUBSCRIBE, and PUBLISH (in cases where QoS > 0) Control Packets MUST contain a non-zero 16-bit Packet Identifier

Or perhaps you would like to allow other control packets to use the identifier 0 and the fix should conditionally appear somewhere else?