[media] Fix double free of video_device in mem2mem_testdev

video_device is already being freed in video_device.release callback on
release.

Signed-off-by: Pawel Osciak <pawel@osciak.com>
Reported-by: Roland Kletzing <devzero@web.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>