Prevent QWindow transient parent loop

Clients may wrongly set the transient parent to the window itself,
causing endless loop when e.g. looking for the top level parent.

Change-Id: Ib23cae3a5576320435ae9b76dd618d1e5ae08b5d
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/gui/kernel/qwindow.cpp

@@ -1279,6 +1279,10 @@ void QWindow::setTransientParent(QWindow *parent)
         qWarning() << parent << "must be a top level window.";
         return;
     }
+    if (parent == this) {
+        qWarning() << "transient parent" << parent << "can not be same as window";
+        return;
+    }
 
     d->transientParent = parent;