Cause of error is that former operator= for SfxPoolItem
copies all but the WhichID from the source, so being quite
(and dangerously) dfferent from Clone() method.
There were quite some places (and will be) that use that
from my POV 'hidden' functionality by creating an Item
with the target-WhichID and then using operator= to assign
data from an Item with different WhichID

Change-Id: Ia7e78d6d1b8b34c9c34e936db10cc850928531d5
Reviewed-on: https://gerrit.libreoffice.org/72399
Tested-by: Jenkins
Reviewed-by: Armin Le Grand <Armin.Le.Grand@me.com>