Since we drop the lock while deleting threads, we need to handle
the queue possibly being accessed and changed by the pool threads
while clear() is running.
Pick-to: 5.15
Fixes: QTBUG-87092
Change-Id: I7611edab90520454278502a58621e299f9cd1f6e
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>