Ensure that RoundRobin.Close() does not panic.

[alecthomas]

This can only occur due to programmer error, but is quite confusing when it does.

Here's an example that triggers it:

package main

import (
	"google.golang.org/grpc"
	"google.golang.org/grpc/naming"
)

type resolver struct {
}

// Resolve creates a Watcher for target.
func (resolver) Resolve(target string) (naming.Watcher, error) {
	return &watcher{addr: target}, nil
}

type watcher struct {
	addr string
}

func (w *watcher) Next() ([]*naming.Update, error) {
	if w.addr == "" {
		select {}
	}
	addr := w.addr
	w.addr = ""
	return []*naming.Update{{Addr: addr}}, nil
}
func (w *watcher) Close() {}

func main() {
	balancer := grpc.RoundRobin(&resolver{})
	_, err := grpc.Dial("127.0.0.1:1234", grpc.WithBalancer(balancer), grpc.WithInsecure())
	if err != nil {
		panic(err)
	}
	err = balancer.Close()
	if err != nil {
		panic(err)
	}
	err = balancer.Close()
	if err != nil {
		panic(err)
	}
	select {}
}

[dfawley]

Thanks for the fix. We will also do this for the grpclb balancer.