model: bugfix - reset the "current_action" after it has been processed

model: bugfix - reset the "current_action" after it has been processed

There are executions under which we may call ModelChecker::check_current_action()
twice in a row without an acutal ModelAction (curren_action) being set in
between. This causes the previous action to be queued twice in our trace
listing.

This can be solved 2 ways:
(1) set current_action to NULL after processing it
(2) move all calls to check_current_action() under the THREAD_READY codepath,
    so that we guarantee that some Thread pushed an Action back to the
    model-checking system

I do both.