scene: Re-order item list integrity checks

Amends 6738473f3d.

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.

src/scene/workspacescene.cpp

@@ -510,12 +510,12 @@ void WorkspaceScene::createStackingOrder()
         }
         std::sort(windowItems.begin(), windowItems.end());
 
-        if (aliveWindowItems != sortedWindowItems) {
-            qFatal("workspaceWindowItems != sortedWindowItems");
-        }
         if (sortedWindowItems != windowItems) {
             qFatal("sortedWindowItems != windowItems");
         }
+        if (aliveWindowItems != sortedWindowItems) {
+            qFatal("workspaceWindowItems != sortedWindowItems");
+        }
     }
 
     for (Item *item : std::as_const(items)) {