rhbz#1071604 don't crash if slide layout configs are missing

Change-Id: I5cbf4ed0683cc5736a45fb980827b1b56bd0c74c
(cherry picked from commit df71f13b80c30cb98e310baf8f6aee11f8cbe81d)
Reviewed-on: https://gerrit.libreoffice.org/9289
Tested-by: Michael Stahl <mstahl@redhat.com>
Reviewed-by: Michael Stahl <mstahl@redhat.com>