media: v4l: Make media_entity_to_video_device() NULL-safe

Make media_entity_to_video_device(NULL) return NULL, instead of an invalid
pointer value.

Signed-off-by: Sakari Ailus <[e-mail]>
Reviewed-by: Laurent Pinchart <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>

include/media/v4l2-dev.h

···
  * media_entity_to_video_device - Returns a &struct video_device from
  *	the &struct media_entity embedded on it.
  *
- * @__entity: pointer to &struct media_entity
+ * @__entity: pointer to &struct media_entity, may be NULL
  */
-#define media_entity_to_video_device(__entity) \
-	container_of(__entity, struct video_device, entity)
+#define media_entity_to_video_device(__entity)				\
+({									\
+	typeof(__entity) __me_vdev_ent = __entity;			\
+									\
+	__me_vdev_ent ?							\
+		container_of(__me_vdev_ent,  struct video_device, entity) : \
+		NULL;							\
+})
 
 /**
  * to_video_device - Returns a &struct video_device from the