QApplication: Fix UB (invalid cast) in notify()

Found by UBSan, which was so shocked that it crashed:

  #6  <signal handler called>
  #7  __dynamic_cast () at ../../../../gcc/libstdc++-v3/libsupc++/dyncast.cc:50
  #8  0x00002b9278fa1c3b in __ubsan::checkDynamicType(void*, void*, unsigned long) () from /opt/gcc/trunk/lib64/libubsan.so.0
  #9  0x00002b9278fa10c3 in HandleDynamicTypeCacheMiss(__ubsan::DynamicTypeCacheMissData*, unsigned long, unsigned long, __ubsan::ReportOptions) () from /opt/gcc/trunk/lib64/libubsan.so.0
  #10 0x00002b9278fa1783 in __ubsan_handle_dynamic_type_cache_miss () from /opt/gcc/trunk/lib64/libubsan.so.0
  #11 0x00002b926c08ab8d in QApplication::notify(QObject*, QEvent*) () at /home/marc/Qt/qt5/qtbase/src/widgets/kernel/qapplication.cpp:3120

(full backtrace originates in tst_QWidget::testDeletionInEventHandlers(),
testing key events).

Fix is simple: just perform the cast before delivering the event.

Change-Id: Ic26e36f47ef57e980c0dba00900927ff39fe6392
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/widgets/kernel/qapplication.cpp

@@ -3178,11 +3178,11 @@ bool QApplication::notify(QObject *receiver, QEvent *e)
                     key->accept();
                 else
                     key->ignore();
-                res = d->notify_helper(receiver, e);
                 QWidget *w = isWidget ? static_cast<QWidget *>(receiver) : 0;
 #ifndef QT_NO_GRAPHICSVIEW
                 QGraphicsWidget *gw = isGraphicsWidget ? static_cast<QGraphicsWidget *>(receiver) : 0;
 #endif
+                res = d->notify_helper(receiver, e);
 
                 if ((res && key->isAccepted())
                     /*