media: v4l2-subdev: Fix error check in v4l2_subdev_get_frame_desc_passthrough()

media: v4l2-subdev: Fix error check in v4l2_subdev_get_frame_desc_passthrough()

Use IS_ERR() and PTR_ERR() to properly handle the error return from
media_pad_remote_pad_unique(), which returns ERR_PTR() on failure but
never NULL. The previous code only checked for NULL, leading to invalid
pointer dereference.

Detected by Smatch:
drivers/media/v4l2-core/v4l2-subdev.c:2588 v4l2_subdev_get_frame_desc_passthrough() warn:
'remote_source_pad' is an error pointer or valid

drivers/media/v4l2-core/v4l2-subdev.c:2595 v4l2_subdev_get_frame_desc_passthrough() error:
'remote_source_pad' dereferencing possible ERR_PTR()

Fixes: a564839e630c ("media: subdev: Add v4l2_subdev_get_frame_desc_passthrough helper")
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Signed-off-by: Chen Ni <nichen@iscas.ac.cn>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>