Bugfix memory exhaustion

This bugfix release fixes a major bug that stayed under the radar. Updating your code is highly recommended!

When hitting low memory conditions and still sending messages, the outbox kept growing but without actual messages. Failures also weren't forwarded to the API. Furthermore, low memory also meant the CONNECT packet could not be allocated so there was no way to connect and process the queue (and thus freeing up memory).

Thanks to @MichaelDvP for discovering and testing.