pkt_sched: Fix oops in htb_delete.

Recent changes introduced a bug in htb_delete(): cl->parent->children
counter update misses checking cl->parent for NULL, which is used for
root classes, so deleting them causes an oops.

Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>