virtio-gpu: check early scanout id

Before accessing the g->scanout array, in order to avoid potential
out-of-bounds access.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 1463653560-26958-2-git-send-email-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
