routing.go: panic: runtime error: index out of range [-1]

panic if no route available:

```
$ go run ./test.go
panic: runtime error: index out of range [-1]

goroutine 1 [running]:
github.com/google/gopacket/routing.(*router).RouteWithSrc(0xc00011c120, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0xc000118220, 0x10, 0x10, ...)
/home/jipan/go/src/github.com/google/gopacket/routing/routing.go:109 +0x6c0
github.com/google/gopacket/routing.(*router).Route(0xc00011c120, 0xc000118220, 0x10, 0x10, 0x4646cd, 0x4c99c0, 0xc000100470, 0xc00004cee8, 0x4ab6f2, 0x4ebc3a, ...)
/home/jipan/go/src/github.com/google/gopacket/routing/routing.go:91 +0x86
main.main()
/home/jipan/go/src/test/test.go:17 +0x131
exit status 2
```
```
$ ip -6 route show
::1 dev lo proto kernel metric 256 pref medium
fe80::/64 dev wlp3s0 proto kernel metric 256 pref medium
fe80::/64 dev wlp3s0 proto kernel metric 600 pref medium

```
Testing code:
```
package main

import (
"fmt"
"log"
"net"

"github.com/google/gopacket/routing"
)

func main() {
ip := net.IP{0x20, 0x01, 0x48, 0x60, 0x00, 0x20, 0x01, 0, 0, 0, 0, 0, 0, 0, 0, 0x68}
router, err := routing.New()
if err != nil {
log.Fatal("routing error:", err)
}
iface, gw, src, err := router.Route(ip)
if err != nil {
log.Fatal("route error:", err)
return
}

fmt.Printf(" iface: %+v\n gw: %v\n src: %v\n", iface, gw, src)
}
```

It works if route exists:
```
$ ip -6 route show
::1 dev lo proto kernel metric 256 pref medium
2607:fb90:9c3e:1a8f::/64 dev wlp3s0 proto ra metric 600 pref medium
fe80::/64 dev wlp3s0 proto kernel metric 256 pref medium
fe80::/64 dev wlp3s0 proto kernel metric 600 pref medium
default via fe80::8a9:77cc:b8fe:4b37 dev wlp3s0 proto ra metric 600 pref medium
```
```
$ go run ./test.go
iface: &{Index:3 MTU:1500 Name:wlp3s0 HardwareAddr:d4:6a:6a:df:90:d7 Flags:up|broadcast|multicast}
gw: fe80::8a9:77cc:b8fe:4b37
src: 2607:fb90:9c3e:1a8f:70c1:e8e6:5f32:fcdf
```