TaskContext: fixed zero pointer fault

The our_act could be nil if there is a parent task given for execution.
In this case our_act->stop() will defnilty fail.
SlaveActivity

Fixed bug in which case operations within Task's were not called anymore
if the task is not running, and a slave of another one.
Merge pull request #35 from goldhoorn/slave_acttivity_fix

Slave acttivity fix