being called from the destructor of a global object when the state object
    the unregister_thread_end_handler() function uses has already been destroyed.
  - Changed the thread pool so that it only gets destroyed if there aren't any
    outstanding threads when the program ends.  If there are threads still
    running user tasks then the thread pool never attempts to destruct.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402860