Fix path traversal issue when extracting an .okular file

Summary:
With specially crafted .okular files you can trick okular to create temporary files outside the temporary folder

We fix that by making sure the file doesn't have folders since the ones we create don't

BUGS: 398096

Subscribers: okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D15192