Fixes CVE-2017-15232, which looks rather minor.

Reviewed-on: https://gerrit.libreoffice.org/68091
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>
(cherry picked from commit e0b96b98)
Reviewed-on: https://gerrit.libreoffice.org/68109Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>

Change-Id: Icffb0c5160bef79577431a02eb10ed9492e01d11