media: v4l: subdev: Make link validation safer

Link validation currently accesses invalid pointers if the link passed to
it is not between two sub-devices. This is of course a driver bug.

Ignore the error but print a warning message, as this is how it used to
work previously.

Fixes: a6b995ed ("media: subdev: use streams in v4l2_subdev_link_validate()")
Reported-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>