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

In the Linux kernel, the following vulnerability has been resolved:

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().

[Sakari Ailus: Reword the subject and commit messages slightly.]