Do not leave unreachable TableColumnUndo/TableRowUndo in case of exceptions

... which might be a memory leak otherwise.

Change-Id: I5e911ba815c3df519365236043fccc7ff21c60b4
Reviewed-on: https://gerrit.libreoffice.org/9979
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>