backmp11: State machine processes events although it is not running

[chandryan]

Calls to process_event in backmp11 are missing a check whether the state machine is running, meaning the state machine seems to be able to process events prior to calling start. However, other functionalities like the active state visitor apply this check. This leads to certain functionalities like deferred events or the history not working properly.

Fix this behavior by ensuring events are not processed as long as the state machine is not running.