Out of order interface error while creating new router

Getting `out of order iface` error from `routing.New`. I can see that the error is coming from this [line](https://github.com/google/gopacket/blob/7c3626525cc0816fa61561fe84cf19e8cbcbe14b/routing/routing.go#L219). But I don't understand why the interface index needs to match with for-loop index.

On my machine the interfaces are listed non-sequentially
```
$ ip addr
1: lo: mtu 65536 qdisc noqueue qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth0: mtu 1500 qdisc mq qlen 1000
link/ether dc:a6:32:6a:e3:b0 brd ff:ff:ff:ff:ff:ff
inet 10.0.0.2/29 brd 10.0.0.7 scope global eth0
valid_lft forever preferred_lft forever
inet6 fe80::dea6:32ff:fe6a:e3b0/64 scope link
valid_lft forever preferred_lft forever
5: wg0: mtu 1420 qdisc noqueue qlen 1000
link/[65534]
inet 10.0.0.1/32 scope global wg0
valid_lft forever preferred_lft forever
```

Can someone explain why the implementation expects interface index to be sequential?