Fix crash in somewhat faulty QGraphicsProxyWidget unit test.

The crash is deep inside QGraphicsSceneIndex, which calls boundingRect()
on the item that is being destroyed. The vtable is busted, resulting in
a pure virtual function call. There's a more proper fix for this lying
around somewhere but in this particular case we can get the test to
not crash by guarding based on whether the item has a cursor set. This
also happens to speed up QGraphicsItem destruction a bit so I figured
it's a win-win situation to fix it. This case will still crash if the
item actually had a cursor set but that makes the case even more narrow.

Generally speaking, creating objects partially on the stack and
partially on the heap, mixing parent/child relationships and then
deleting one of the heap objects is quite sketchy and I doubt it happens
much outside of this unit test.

Change-Id: I25393d2cafb1256269ab6681519bd554cc380bfd
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/widgets/graphicsview/qgraphicsitem.cpp

@@ -2147,6 +2147,8 @@ bool QGraphicsItem::hasCursor() const
 */
 void QGraphicsItem::unsetCursor()
 {
+    if (!d_ptr->hasCursor)
+        return;
     d_ptr->unsetExtra(QGraphicsItemPrivate::ExtraCursor);
     d_ptr->hasCursor = 0;
     if (d_ptr->scene) {