Panic on Handle /:foo:bar

[rogpeppe]

The following program panics with "slice bounds out of range" error. This
case should be caught and a more understandable message issued instead.

package main

import (
    "github.com/julienschmidt/httprouter"
    "net/http"
)

func main() {
    r := httprouter.New()
    r.Handle("GET", "/:foo:bar/oops", func(http.ResponseWriter, *http.Request, httprouter.Params) {})
}

The panic is as follows:

panic: runtime error: slice bounds out of range

goroutine 1 [running]:
github.com/julienschmidt/httprouter.(*node).insertChild(0xc20805c120, 0x1, 0x6bd190, 0xe, 0x72ea70)
    /home/rog/src/go/src/github.com/julienschmidt/httprouter/tree.go:217 +0x57a
github.com/julienschmidt/httprouter.(*node).addRoute(0xc20805c060, 0x6bd190, 0xe, 0x72ea70)
    /home/rog/src/go/src/github.com/julienschmidt/httprouter/tree.go:184 +0xac
github.com/julienschmidt/httprouter.(*Router).Handle(0xc208030f70, 0x6af1d0, 0x3, 0x6bd190, 0xe, 0x72ea70)
    /home/rog/src/go/src/github.com/julienschmidt/httprouter/router.go:219 +0x1ae
main.main()
    /home/rog/src/tst.go:10 +0x9a

[julienschmidt]

Even worse: /:foo:bar currently works without panicking at all and the router creates nodes for both :foo:bar and :bar!
I'm working on a fix.