fdo#72468 avoid out of bounds array access

Change-Id: I2bccdf1aaeb290a3cb8f76cd831a419e1776e92f
Reviewed-on: https://gerrit.libreoffice.org/7775
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>