Crashes in SchedulerMutex::acquire if destroyed from __run_exit_handlers

Change-Id: Ie8bbb399183d118d92c94a40f55cc9c650baf742
Reviewed-on: https://gerrit.libreoffice.org/41629Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>