Changed task queue management a little for ZOOM C.

Changed task queue management a little for ZOOM C.
When a diagnostic is produced/received (error condition), the task queue is
cleared immediately. In earlier versions the task queue was cleared when
a new task was inserted in the queue. The effect was that a newly inserted task
was _immediately_ removed and never fired. Hence tasks were silenty
forgotten. The error code is still cleared when a new task is inserted
in the queue.