There's reams of code already preventing a close during save
(SfxSaveGuard), but this looks equally effective. Avoids
SfxBaseModel::dispose() pulling the rug under us, when
SfxBaseModel::postEvent_Impl() wants to access parts of the
interface containers during notification.

Change-Id: Iace1f9922c47a97ab2e798b577c6aec8a729da48
Reviewed-on: https://gerrit.libreoffice.org/48709Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>