The short-lived SwBorderAttrAccess is manually deleted
which is not exception safe and can potentially leak.
This wraps it in unique_ptr.

Change-Id: Ib45c1c36214583e0bf205231f9f793e023d106c7
Reviewed-on: https://gerrit.libreoffice.org/19701Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>