[media] videobuf-dma-contig: Use NULL instead of plain integer

Fixes the following sparse warning:
drivers/media/video/videobuf-dma-contig.c:59:46:
warning: Using plain integer as NULL pointer

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
1 file changed