Default MQTT Payload Parser can't handle empty messages

[fooker]

MQTT supports sending messages without a payload (the remaining bytes after the variable headers are zero). If such a message is received by the MqttClient and processed by the default MqttPayloadParser, the client dies and does not accept any further messages.

At a first sight, it looks like malloc is called with message->publish.content.length which is 0 in such a case. Thus leading malloc to return nullptr and the following sanity-check to fail.

[slaff]

@fooker Can you send a PR with your suggested fix?

[fooker]

It looks like fixing the payloadParser is not enough. The on_data_payload and on_data_end callbacks will be called way to late (the are called if a follow-up message is received).

I've opened an associated issue in the according library: slaff/mqtt-codec#3

[fooker]

@slaff If slaff/mqtt-codec#4 gets merged, there is no need for change in Sming.

Another way to solve this is not to skip the callbacks on empty messages, but make them all be called with zero length. In such a case, I will come up with a PR.

[fooker]

@slaff will this be back-ported to a 4.0.x release?