Fix decrement index and not iterator as discussed with David

Summary: Fix decrement index and not iterator

Reviewers: dfaure, anthonyfieroni

Reviewed By: dfaure, anthonyfieroni

Subscribers: anthonyfieroni, kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D25902