Fix empty screen with recent freerdp versions

With recent freerdp versions, gtk-frdp shows just an empty screen and prints the following:

[15:33:45:659] [11955:11955] [ERROR][com.freerdp.core.transport]
- transport_check_fds: transport->ReceiveCallback() - -4

This is caused by the wrong return value from frdp_end_paint. Let's fix that return value to make it work again.

gnome-boxes#280 (closed)