and put an assert in SfxPoolItem::SetWhich() so nothing new
creeps in.

Change-Id: I6497650fa61ffb2b6941ffff2d471c8f117be1df
Reviewed-on: https://gerrit.libreoffice.org/24324Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>