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>
(cherry picked from commit 955c5539a1ea5e971f111989d6c5bec11d936416)
Signed-off-by: Michael Stahl <mstahl@redhat.com>