It uses random-access iterators, so just use O(1) increments

Change-Id: I9f80789d0bc03184d346c6814fd015bc06876acd
Reviewed-on: https://gerrit.libreoffice.org/42606Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>