Reset in qoffscreensurface when closing app when using multiple monitors

The qoffscreensurface::setScreen crashes in create if application is
closing. This happens in multimonitor setup when one of the screens is
already destroyed and qoffscreensurface handles the screenDestroyed
signal. In this case the QGuiApplication::primaryScreen returns non-null
surface and selects the codepath that recreates the surface. The create
function calls the qGuiApp->thread function, which crashes since the
application instance has already been set to nullptr.