Android: Fix crash when hitting Back button to quit

On Android, it's possible to reach the condition when window == 0
if you hit the Back key, since the previous bail-out is disabled
for this case. To avoid a null-pointer dereference, an extra
check is required.

Change-Id: Ic898cd82dd6e52c24505dd2248c98efcefc15b1c
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>

src/gui/kernel/qguiapplication.cpp

@@ -1557,7 +1557,7 @@ void QGuiApplicationPrivate::processKeyEvent(QWindowSystemInterfacePrivate::KeyE
             ) {
         return;
     }
-    if (window->d_func()->blockedByModalWindow) {
+    if (window && window->d_func()->blockedByModalWindow) {
         // a modal window is blocking this window, don't allow key events through
         return;
     }