[media] v4l: subdev: tolerate null in media_entity_to_v4l2_subdev

Return NULL, if a null entity is parsed for it's v4l2_subdev

Currently, the callers of media_entity_to_v4l2_subdev() need to
make sure that the subdev is non-NULL, for example, when it is
obtained from media_entity_remote_pad().

As this is a recurring pattern, add the check at the macro in order
to avoid additional checks at the callers.
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

include/media/v4l2-subdev.h

@@ -827,7 +827,14 @@ struct v4l2_subdev {
 };
 
 #define media_entity_to_v4l2_subdev(ent)				\
-	container_of(ent, struct v4l2_subdev, entity)
+({									\
+	typeof(ent) __me_sd_ent = (ent);				\
+									\
+	__me_sd_ent ?							\
+		container_of(__me_sd_ent, struct v4l2_subdev, entity) :	\
+		NULL;							\
+})
+
 #define vdev_to_v4l2_subdev(vdev) \
 	((struct v4l2_subdev *)video_get_drvdata(vdev))