[PATCH] nexthop_compare_node: fix NULL iface dereference

Hello, I am proposing a fix for a segfault encountered in bird6, in the nexthop_compare_node function. This patch adds checks for validating that x->iface and y->iface are not null before accessing their index field. I initially proposed a fix on calico's bird fork : https://github.com/projectcalico/bird/pull/117 but I was asked to propose it to the official repo. Please find the full patch attached. Best regards, Mathieu.