avoid buffer overflow by using wrong buffer

Found by Asan.

(regression from CWS mba34issues01)

Change-Id: Ia7159c9bed9d7f823448acd02e18568a5f3f2093
Reviewed-on: https://gerrit.libreoffice.org/10717
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>