Fix fdo#71423 - crash while editing Impress tables.

Band-aid fix for this nasty crasher, seems sometimes the
family item is not found.

Change-Id: I7f78d59cbf2c9595ad512fa7a78370629924cd06
(cherry picked from commit 058cfa72dcdd91df302be45f7fc59b0d961a420b)
Signed-off-by: Andras Timar <andras.timar@collabora.com>
(cherry picked from commit 4128c63d655869e0ab7befde2c46761d8e1b273f)
Reviewed-on: https://gerrit.libreoffice.org/9123
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>
Reviewed-by: David Tardon <dtardon@redhat.com>