scene: Re-order item list integrity checks
Amends 6738473f.
It's better to check whether Item::childItems() has the same items as Item::sortedChildItems() first because it gives more context about the integrity of the lists.
If Item::sortedChildItems() is corrupted but Item::childItems() is fine, we may miss that important detail because "aliveWindowItems != sortedWindowItems" comes first.
(cherry picked from commit 1df73874)
Co-authored-by: Vlad Zahorodnii vlad.zahorodnii@kde.org