net/sched: fix pointer check in gen_handle

Fixes sparse warning about pointer in gen_handle:
net/sched/cls_rsvp.h:392:40: warning: Using plain integer as NULL pointer

Fixes: 8113c095

 ("net_sched: use void pointer for filter handle")
Signed-off-by: Josh Hunt <johunt@akamai.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>