KDirectoryContentsCounter: do not delete currently active worker objects

Before this patch, the destructor of KDirectoryContentsCounter might
delete the worker object, which lives in another thread, while one of
its methods was still being executed. This could cause a crash. Only if
the destroyed KDirectoryContentsCounter was the last one, the worker
thread was stopped, and the destructor waited until all workers are
done.

BUG: 332767
FIXED-IN: 4.13.0
REVIEW: 117209