media: vivid: fix error handling of kthread_run

kthread_run returns an error pointer, but elsewhere in the code
dev->kthread_vid_cap/out is checked against NULL.

If kthread_run returns an error, then set the pointer to NULL.

I chose this method over changing all kthread_vid_cap/out tests
elsewhere since this is more robust.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reported-by:  <syzbot+53d5b2df0d9744411e2e@syzkaller.appspotmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>