KTar::openArchive: Don't assert if file has two root dirs

Summary:
I was wondering if adding this check in KArchive::setRootDir
instead but i ended doing it here since i think an assert in setRootDir is
valid for when C++ users of that class do it wrong.

Reviewers: apol, dfaure

Reviewed By: apol, dfaure

Subscribers: dfaure, apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22786