If it's disposed during document destruction it still retained pointers
to SfxItemSet and SfxStyleSheetBasePool.

Change-Id: I7b986b4fe5ca466f0fa86a99b744f5e2f85e08be
(cherry picked from commit 8904bf48)
Reviewed-on: https://gerrit.libreoffice.org/16866Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>