Cocoa: Prevent crash on QWindow delete.

Clear the QCocoaWindow pointer stored on the QNSWindowHelper
when detaching from the platform window. This makes
sure callbacks from cocoa does not try access deleted
Qt window objects.

Task-number: QTBUG-39141
Change-Id: I4672eae92940dcbd59a2848e56f94b50a828bbf6
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>

src/plugins/platforms/cocoa/qcocoawindow.mm

@@ -182,6 +182,7 @@ static bool isMouseEvent(NSEvent *ev)
 
 - (void)detachFromPlatformWindow
 {
+    _platformWindow = 0;
     [self.window.delegate release];
     self.window.delegate = nil;
 }