Track QPlatformScreen -> QScreen using QPointer

Allows the QPlatformScreen to verify that the QScreen is alive before
sending events to QPA for the given screen (which will assert if the
screen is being destroyed).

Change-Id: Ie77674fead3e0a4d4f6fedbf1f7f3c98364c7485
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>

src/gui/kernel/qplatformintegration.cpp

@@ -461,7 +461,6 @@ void QPlatformIntegration::destroyScreen(QPlatformScreen *screen)
 {
     QGuiApplicationPrivate::screen_list.removeOne(screen->d_func()->screen);
     delete screen->d_func()->screen;
-    screen->d_func()->screen = Q_NULLPTR;
     delete screen;
 }
 

src/gui/kernel/qplatformscreen_p.h

@@ -52,7 +52,7 @@ class QScreen;
 class QPlatformScreenPrivate
 {
 public:
-    QScreen *screen;
+    QPointer<QScreen> screen;
 };
 
 QT_END_NAMESPACE

src/plugins/platforms/ios/qiosviewcontroller.mm

@@ -337,6 +337,9 @@
     if (!isQtApplication())
         return;
 
+    if (!m_screen->screen())
+        return;
+
     // For now we only care about the main screen, as both the statusbar
     // visibility and orientation is only appropriate for the main screen.
     if (m_screen->uiScreen() != [UIScreen mainScreen])