do not crash when pasting impress page into another impress page (cp#1000067)

Put an Impress page into clipboard, create new document, click 'Click to add
title', paste -> crash.
Introduced by 5aee0c13cc62a81a38d4132db7972dc301c21b3d.

Change-Id: Ibe61a780244ce98205e5ae3964971d7d75e30870
(cherry picked from commit dffcd67a9a9865c9eca9756eea4e85445b50a15e)
Reviewed-on: https://gerrit.libreoffice.org/9178
Reviewed-by: Andras Timar <andras.timar@collabora.com>
Tested-by: Andras Timar <andras.timar@collabora.com>