nexthop: off by one in nexthop_mpath_select()

The nhg->nh_entries[] array is allocated in nexthop_grp_alloc() and it
has nhg->num_nh elements so this check should be >= instead of >.

Fixes: 430a0491

 ("nexthop: Add support for nexthop groups")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>