Only attempt to flush queue if the underlying worker pool is not finished (#18593)

Only attempt to flush queue if the underlying worker pool is not finished (#18593)

* Only attempt to flush queue if the underlying worker pool is not finished

There is a possible race whereby a worker pool could be cancelled but yet the
underlying queue is not empty. This will lead to flush-all cycling because it
cannot empty the pool.

Signed-off-by: Andrew Thornton <art27@cantab.net>
* Apply suggestions from code review

Co-authored-by: Gusted <williamzijl7@hotmail.com>
Co-authored-by: Gusted <williamzijl7@hotmail.com>