media: nxp: imx8-isi: Check whether crossbar pad is non-NULL before access

When translating source to sink streams in the crossbar subdev, the
driver tries to locate the remote subdev connected to the sink pad. The
remote pad may be NULL, if userspace tries to enable a stream that ends
at an unconnected crossbar sink. When that occurs, the driver
dereferences the NULL pad, leading to a crash.

Prevent the crash by checking if the pad is NULL before using it, and
return an error if it is.

Cc: stable@vger.kernel.org # 6.1
Fixes: cf21f328 ("media: nxp: Add i.MX8 ISI driver")
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/20231201150614.63300-1-marex@denx.de


Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>