Resolves: rhbz#1121341 crash on closing shrunk validity window

Change-Id: I5fc1d6b909b3281ec3799b60bc3fd7b5f1163a75
Reviewed-on: https://gerrit.libreoffice.org/10439
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>