To join a thread, the mutex must be released - another thread
in pushTask() could add a new thread to maWorkers at that point,
which must of course not be deleted.

Avoid the problem by transferring ownership of the to-be-deleted
threads to the calling thread.

(regression from bdaa13a8)

Change-Id: I9d4fcfe4cb46a336586b5663934a12d47b2d8ccb