Don't try to reposition the ScrollView when we get a faulty height

Summary: BUG: 401960

Test Plan: Tried as explained in the bug, can't see the issue anymore

Reviewers: #kirigami, mart

Reviewed By: #kirigami, mart

Subscribers: plasma-devel

Tags: #kirigami

Differential Revision: https://phabricator.kde.org/D17562