fdo#73464 - get relation BSTR allocation right.

Change-Id: Iceffca157643c2d73f581b655b39e09486c83897
Reviewed-on: https://gerrit.libreoffice.org/7700
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>