[media] v4l: omap_vout: Don't free buffers if they haven't been allocated

The VRFB buffers are freed when the device is closed even if they
haven't been allocated by a call to VIDIOC_REQBUFS, resulting in a
crash. Fix it by not trying to free buffers that are not allocated.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>