desktop-shell: fix the crash while clicking TRANSIENT_INACTIVE window

It is pretty easy to replicate this bug by involving a Qt Window with Qt::WindowTransparentForInput flag.

int main(int argc, char *argv[]) { QApplication a(argc, argv); MainWindow w; w.show();

QDialog d1(&w, Qt::WindowTransparentForInput);
d1.show();

return a.exec();

}

Click d1 dialog, weston will crash due to NULL==shruf.

Signed-off-by: Barry Song barry.song@navico.com