QNX: Avoid crashing on exit when have a cover window

QQnxScreen destructor was leaving the cover window to a reference
to it leading to a crash when the cover window itself was destroyed.

Change-Id: Id82097d54fea79b0b9ec1cef439bd704da9ca453
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Bernd Weimer <bweimer@blackberry.com>
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>

src/plugins/platforms/qnx/qqnxscreen.cpp

@@ -200,6 +200,9 @@ QQnxScreen::~QQnxScreen()
     Q_FOREACH (QQnxWindow *childWindow, m_childWindows)
         childWindow->setScreen(0);
 
+    if (m_coverWindow)
+        m_coverWindow->setScreen(0);
+
     delete m_cursor;
 }