ScheduledWorkItem::ScheduleClear: remove item from runnable queue

The existing behavior is unexpected: if the work item is already on the
runnable queue, it will still be triggered after a call to ScheduleClear().
This can lead to race conditions.

platforms/common/px4_work_queue/ScheduledWorkItem.cpp

@@ -66,7 +66,9 @@ ScheduledWorkItem::ScheduleOnInterval(uint32_t interval_us, uint32_t delay_us)
 void
 ScheduledWorkItem::ScheduleClear()
 {
+	// first clear any scheduled hrt call, then remove the item from the runnable queue
 	hrt_cancel(&_call);
+	WorkItem::ScheduleClear();
 }
 
 void