Don't attempt to publish MQTT messages unless/until connected to broker

For cases where an mqtt struct is shared between threads without concurrency
control. I'm making this improvement in light of research into issue #781,
though I don't expect this change to fix the reported issue.

The reported error seems to come from the event thread (publishing node
status upon starting to run the "DAILY" event) when a broker connection was
not successful, however the reporter (Nelgin) may not have had debug-level
logging turned on, so didn't capture the successful broker-connect log
message. I think the broker connection *was* successful and perhaps then
terminated by the broker ("due to protocol error"?).