get_user_pages() can return a negative error code when it fails. Set
buf->npages to 0 in that case, to prevent isp_video_buffer_cleanup()
from releasing invalid pages.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>