Bug#862357: pidx % serv->sv_nrpools causes segfault

[Nye Liu <nyet@nyet.org>]

At net/sunrpc/svc.c line 355 svc_pool_for_cpu(), srv->sv_nrpools can be 
zero:

        return &serv->sv_pools[pidx % serv->sv_nrpools];