QVncScreen: fix crash on disconnect client did not request cursor

Change-Id: I758c79d87bd239b6fde9bae4e97c5b31450fa813
Pick-to: 5.15 5.12
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/plugins/platforms/vnc/qvncscreen.cpp

@@ -139,6 +139,9 @@ void QVncScreen::enableClientCursor(QVncClient *client)
 void QVncScreen::disableClientCursor(QVncClient *client)
 {
 #if QT_CONFIG(cursor)
+    if (!clientCursor)
+        return;
+
     uint clientCount = clientCursor->removeClient(client);
     if (clientCount == 0) {
         delete clientCursor;