[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>