[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>

drivers/media/video/videobuf-dma-contig.c

@@ -56,7 +56,7 @@ static int __videobuf_dc_alloc(struct device *dev,
 				dev_err(dev, "dma_map_single failed\n");
 
 				free_pages_exact(mem->vaddr, mem->size);
-				mem->vaddr = 0;
+				mem->vaddr = NULL;
 				return err;
 			}
 		}