Under certain circumstances, if a stream's task is first woken up
(eg: I/O event) then notified of the availability of a buffer it
was waiting for via stream_res_wakeup(), this second event is lost
because the flags are only merged after seeing that the task is
running. At the moment it seems that the TASK_WOKEN_RES event is
not explicitly checked for, but better fix this before getting
reports of lost events.
This fix removes this "task running" test which is properly
performed in task_wakeup(), while the flags are properly merged.
It must be backported to 1.7 and 1.6.
ff4491726f