Cocoa: Fix crash on screen disconnect.

Maintain virtual siblings list on screen deletion.

QCocoaIntegration::updateScreens() has a loop which
will delete all non-current QScreen objects using
QPlatformIntegration::destroyScreen().

destroyScreen() vill eventually call QWindowPrivate::
setTopLevelScreen() which accesses the virtual siblings
list for the deleted screen.

This can cause a stale pointer access if the virtual
screen list is not up to date, especially when disconnecting
two screens at the same time.

Change-Id: Ia6b9d01edf8e5eea25b64604a2b3b28b173125f7
Task-number: QTBUG-48275
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@theqtcompany.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>

src/plugins/platforms/cocoa/qcocoaintegration.mm

@@ -422,14 +422,18 @@ void QCocoaIntegration::updateScreens()
         }
         siblings << screen;
     }
+
+    // Set virtual siblings list. All screens in mScreens are siblings, because we ignored the
+    // mirrors. Note that some of the screens we update the siblings list for here may be deleted
+    // below, but update anyway to keep the to-be-deleted screens out of the siblings list.
+    foreach (QCocoaScreen* screen, mScreens)
+        screen->setVirtualSiblings(siblings);
+
     // Now the leftovers in remainingScreens are no longer current, so we can delete them.
     foreach (QCocoaScreen* screen, remainingScreens) {
         mScreens.removeOne(screen);
         destroyScreen(screen);
     }
-    // All screens in mScreens are siblings, because we ignored the mirrors.
-    foreach (QCocoaScreen* screen, mScreens)
-        screen->setVirtualSiblings(siblings);
 }
 
 QCocoaScreen *QCocoaIntegration::screenAtIndex(int index)