Resolves: fdo#77516 sub-grid shouldn't be allowed as 0

regression from

commit 3cb6587abbf6888ac9fb3b0cccc9a5aa0dc29b55
Author: Csikós Tamás <csks.tomi@gmail.com>
Date:   Sun Jul 7 13:40:53 2013 +0200
    modern .ui widgetlayout for optgrid

Change-Id: I9e998ce203c16c2303e00347ce6a2992c92cc8dc
(cherry picked from commit 4bd317f845c2f3470c7fc354e478be08c2949e0c)
Reviewed-on: https://gerrit.libreoffice.org/11164
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>