sch_atm: Fix potential NULL deref.

The list_head conversion unearther an unnecessary flow
check.  Since flow is always NULL here we don't need to
see if a matching flow exists already.
Reported-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_atm.c

@@ -255,10 +255,6 @@ static int atm_tc_change(struct Qdisc *sch, u32 classid, u32 parent,
 			error = -EINVAL;
 			goto err_out;
 		}
-		if (!list_empty(&flow->list)) {
-			error = -EEXIST;
-			goto err_out;
-		}
 	} else {
 		int i;
 		unsigned long cl;