Check if group < LastGroup, as KIconEffect doesn't handle UserGroup anyway

Test Plan:
Run the steps described in the testplan of D12001. Without the patch, the
warning "Illegal icon group: 7" gets displayed, as Okular uses User icons.
Applying the patch fixes this; unit tests still run without failure.

Reviewers: mart, dfaure, mlaurent

Reviewed By: dfaure

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12002