Most of these calls are in assertions, and the ones that aren't should
be guarded by SolarMutex, so the thread safety concerns that caused
unique() to be deprecated don't look relevant, so use use_count(),
which oddly enough isn't deprecated.

Change-Id: Ia166615af6c3ce85145c391752669c7461bd35fb
Reviewed-on: https://gerrit.libreoffice.org/48222Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>