Fix focusproxy-relayed crash in QGraphicsItem destructor.

Makes tst_qgraphicsitem::tst_focusProxyDeletion not crash.

valgrind reported the error when running tst_qgraphicsitem. The crash
was very real; when deleting an item that has another item as a focus
proxy, the proxy still had a reference to the deleted item's focusProxy
pointer. I'm not a huge fan of whitebox testing but thought this crash
justifies a modification of the test to make it fail, instead of just
passing silently with a warning only given by valgrind and friends.

FTR the reason the test doesn't crash hard is that the memory is freed
but not reused within the scope of the test. So the access to the
pointer d_ptr->focusProxy succeeds, it just accesses memory that might
as well have been reclaimed. But this is quite undefined...

Task-number: QTBUG-28321
Change-Id: I2624631f5e5c2a8aa8bd4efe1fc128eba6c61f56
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>

src/widgets/graphicsview/qgraphicsitem.cpp

@@ -1436,6 +1436,7 @@ QGraphicsItem::~QGraphicsItem()
 #endif
 
     clearFocus();
+    setFocusProxy(0);
 
     // Update focus scope item ptr.
     QGraphicsItem *p = d_ptr->parent;

tests/auto/widgets/graphicsview/qgraphicsitem/tst_qgraphicsitem.cpp

@@ -8485,7 +8485,9 @@ void tst_QGraphicsItem::focusProxyDeletion()
 
     rect2 = new QGraphicsRectItem;
     rect->setFocusProxy(rect2);
+    QGraphicsItem **danglingFocusProxyRef = &rect->d_ptr->focusProxy;
     delete rect; // don't crash
+    QVERIFY(!rect2->d_ptr->focusProxyRefs.contains(danglingFocusProxyRef));
 
     rect = new QGraphicsRectItem;
     rect->setFocusProxy(rect2);