[media] v4l2-ctrls.c: add NULL check

Check that the control whose events we want to delete is still there.

Normally this will always be the case, but I am not 100% certain if
there aren't any corner cases when a device is forcibly unbound.

In any case, this will satisfy static checkers and simply make it more
robust.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reported-by: Shaobo <shaobo@cs.utah.edu>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/v4l2-core/v4l2-ctrls.c

@@ -3367,6 +3367,9 @@ static void v4l2_ctrl_del_event(struct v4l2_subscribed_event *sev)
 {
 	struct v4l2_ctrl *ctrl = v4l2_ctrl_find(sev->fh->ctrl_handler, sev->id);
 
+	if (ctrl == NULL)
+		return;
+
 	v4l2_ctrl_lock(ctrl);
 	list_del(&sev->node);
 	v4l2_ctrl_unlock(ctrl);