this is similar to tdf#87199, in this case the accessibiles for the merged
cells are not visible so not removed when their frame is deleted, but remain
in the cache pointing to invalid frames.

Change-Id: Ibc5b9f27541683b8f3604839fa3d1431380a4039
Reviewed-on: https://gerrit.libreoffice.org/54808Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>