Don't crash on unstarted table

Change-Id: I68e596ea37133c89206333e8ca8aa3602878d2fc
(cherry picked from commit 6f52d472393e4d92c0a4d988f28312edaeb5775c)
Reviewed-on: https://gerrit.libreoffice.org/7617
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>