AncestorsWalker panics if we try to stop at the last vertex

[gaborszakacs]

Reproduction based on the original usage example, but modifying the exit criteria:

package main

import (
	"fmt"

	"github.com/heimdalr/dag"
)

func main() {
	dag := dag.NewDAG()

	v1, _ := dag.AddVertex("1")
	v2, _ := dag.AddVertex("2")
	v3, _ := dag.AddVertex("3")
	v4, _ := dag.AddVertex("4")
	v5, _ := dag.AddVertex("5")
	_ = dag.AddEdge(v1, v2)
	_ = dag.AddEdge(v2, v3)
	_ = dag.AddEdge(v2, v4)
	_ = dag.AddEdge(v4, v5)

	var ancestors []interface{}
	vertices, signal, _ := dag.AncestorsWalker(v5)
	for v := range vertices {
		ancestors = append(ancestors, v)
		// v2 was modified to v1
		if v == v1 {
			signal <- true
			break
		}
	}
	fmt.Printf("%v", ancestors)
}

Results in

panic: send on closed channel

I'm happy to contribute, but I'm not quite sure how to resolve this while maintaining the current API contract.