Race condition in handling of `cmd`

[wrboyce]

==================
WARNING: DATA RACE
Write at 0x00c4201ba208 by goroutine 32:
  os/exec.(*Cmd).Wait()
      /usr/local/Cellar/go/1.9.2/libexec/src/os/exec/exec.go:450 +0x163
  github.com/resin-io/sshproxy.(*Server).handleRequests.func1.1()
      /Users/wrboyce/Dev/resin/.go/src/github.com/resin-io/sshproxy/sshproxy.go:249 +0x4c

Previous read at 0x00c4201ba208 by goroutine 30:
  github.com/resin-io/sshproxy.(*Server).handleRequests.func1()
      /Users/wrboyce/Dev/resin/.go/src/github.com/resin-io/sshproxy/sshproxy.go:269 +0x29b

Goroutine 32 (running) created at:
  github.com/resin-io/sshproxy.(*Server).handleRequests.func1()
      /Users/wrboyce/Dev/resin/.go/src/github.com/resin-io/sshproxy/sshproxy.go:248 +0x8a

Goroutine 30 (finished) created at:
  github.com/resin-io/sshproxy.(*Server).handleRequests()
      /Users/wrboyce/Dev/resin/.go/src/github.com/resin-io/sshproxy/sshproxy.go:246 +0xb0d
  github.com/resin-io/sshproxy.(*Server).handleChannels.func1()
      /Users/wrboyce/Dev/resin/.go/src/github.com/resin-io/sshproxy/sshproxy.go:176 +0x8a
==================

Reproducible using:

package sshproxy_test

import (
	"net"
	"time"
	"testing"

	"golang.org/x/crypto/ssh"
	"github.com/resin-io/sshproxy"
)

func TestRace(t *testing.T) {
	server, err := sshproxy.New("/tmp", "/bin/bash", false, nil, nil, nil)
	if err != nil {
		t.Errorf("error calling sshproxy.New :( %q", err)
	}
	port := "12345"
	go server.Listen(port)

	config := &ssh.ClientConfig{
		User: "user",
		Auth: []ssh.AuthMethod{
			ssh.Password("password"),
		},
		HostKeyCallback: func(hostname string, remote net.Addr, key ssh.PublicKey) error {
			return nil
		},
	}
	for i := 0; i < 10; i++ {
		client, err := ssh.Dial("tcp", "localhost:12345", config)
		if err != nil {
			t.Errorf("Cannot connect to server :( %q", err)
		}
		session, err := client.NewSession()
		if err != nil {
			t.Errorf("Cannot create session :( %q", err)
		}
		time.Sleep(time.Second)
		_, err = session.SendRequest("exec", false, []byte{0, 0, 0, 4, 't', 'e', 's', 't'})
		if err != nil {
			t.Errorf("Cannot send exec request :( %q", err)
		}
		time.Sleep(time.Duration(i * 100) * time.Millisecond)
		client.Close()
	}
}