fdo#79761: parse BlockList.xml only once

Change-Id: I3cfc5b66ee73b0e4d07a84c8255c5a006e4fbb25
Reviewed-on: https://gerrit.libreoffice.org/12210
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>