media: v4l: async: Fix NULL pointer dereference in adding ancillary links

media: v4l: async: Fix NULL pointer dereference in adding ancillary links

In v4l2_async_create_ancillary_links(), ancillary links are created for
lens and flash sub-devices. These are sub-device to sub-device links and
if the async notifier is related to a V4L2 device, the source sub-device
of the ancillary link is NULL, leading to a NULL pointer dereference.
Check the notifier's sd field is non-NULL in
v4l2_async_create_ancillary_links().

Fixes: aa4faf6eb271 ("media: v4l2-async: Create links during v4l2_async_match_notify()")
Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
[Sakari Ailus: Reword the subject and commit messages slightly.]
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>