fdo#78535: Don't process borders of 0 width.

Or they would get drawn for real.

Change-Id: Iec24e70a80ff8aa1ef7512e71249a99243297c4e
(cherry picked from commit 224b235971a01971de626d38ccc8506d0a55771b)
Reviewed-on: https://gerrit.libreoffice.org/9323
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>