zauberhaus/rest2dhcp: routing/routing.go; 14 LoC

Found a possible issue in [zauberhaus/rest2dhcp](https://www.github.com/zauberhaus/rest2dhcp) at [routing/routing.go](https://github.com/zauberhaus/rest2dhcp/blob/f4b631d7f25e00113f3edbdb05e2e818e5a1a3a0/routing/routing.go#L107-L120)

Below is the message reported by the analyzer for this snippet of code. Beware that the analyzer only reports the first
issue it finds, so please do not limit your consideration to the contents of the below message.

> reference to f is reassigned at line 109

[Click here to see the code in its original context.](https://github.com/zauberhaus/rest2dhcp/blob/f4b631d7f25e00113f3edbdb05e2e818e5a1a3a0/routing/routing.go#L107-L120)

Click here to show the 14 line(s) of Go which triggered the analyzer.

```go
for i, f := range r.ifaces {
if f.Index == ifaceIndex {
iface = &f

switch {
case dst.To4() != nil:
preferredSrc = r.addrs[i].v4
case dst.To16() != nil:
preferredSrc = r.addrs[i].v6
}

break
}
}

```

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See the descriptions of the classifications [here](https://github.com/github-vet/rangeclosure-findings#how-can-i-help) for more information.

commit ID: f4b631d7f25e00113f3edbdb05e2e818e5a1a3a0