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>

drivers/media/platform/vivid/vivid-kthread-cap.c

@@ -873,8 +873,11 @@ int vivid_start_generating_vid_cap(struct vivid_dev *dev, bool *pstreaming)
 			"%s-vid-cap", dev->v4l2_dev.name);
 
 	if (IS_ERR(dev->kthread_vid_cap)) {
+		int err = PTR_ERR(dev->kthread_vid_cap);
+
+		dev->kthread_vid_cap = NULL;
 		v4l2_err(&dev->v4l2_dev, "kernel_thread() failed\n");
-		return PTR_ERR(dev->kthread_vid_cap);
+		return err;
 	}
 	*pstreaming = true;
 	vivid_grab_controls(dev, true);

drivers/media/platform/vivid/vivid-kthread-out.c

@@ -244,8 +244,11 @@ int vivid_start_generating_vid_out(struct vivid_dev *dev, bool *pstreaming)
 			"%s-vid-out", dev->v4l2_dev.name);
 
 	if (IS_ERR(dev->kthread_vid_out)) {
+		int err = PTR_ERR(dev->kthread_vid_out);
+
+		dev->kthread_vid_out = NULL;
 		v4l2_err(&dev->v4l2_dev, "kernel_thread() failed\n");
-		return PTR_ERR(dev->kthread_vid_out);
+		return err;
 	}
 	*pstreaming = true;
 	vivid_grab_controls(dev, true);