fix panic when passing empty environment

Fix a potential panic (introduced on [Apr 30](https://github.com/dotcloud/docker/blame/master/pkg/libcontainer/nsinit/init.go#L154), released docker as 0.11.1) when starting containers via API:

```
panic: runtime error: index out of range

goroutine 1 [running]:
runtime.panic(0xa4d6c0, 0x1417cb7)
        /usr/local/go/src/pkg/runtime/panic.c:266 +0xb6
github.com/dotcloud/docker/pkg/libcontainer/nsinit.LoadContainerEnvironment(0xc2100f69a0, 0x59, 0xc210037780)
        /go/src/github.com/dotcloud/docker/pkg/libcontainer/nsinit/init.go:155 +0x126
github.com/dotcloud/docker/pkg/libcontainer/nsinit.Init(0xc2100f69a0, 0xc210037660, 0x59, 0x7fff64970eee, 0x0, ...)
        /go/src/github.com/dotcloud/docker/pkg/libcontainer/nsinit/init.go:35 +0x9a
github.com/dotcloud/docker/daemon/execdriver/native.func·001(0xc210079180, 0xc21001ef90, 0x7fff64970ede)
        /go/src/github.com/dotcloud/docker/daemon/execdriver/native/driver.go:49 +0x308
github.com/dotcloud/docker/sysinit.executeProgram(0xc210079180, 0xc210079180, 0xadef80)
        /go/src/github.com/dotcloud/docker/sysinit/sysinit.go:18 +0xbc
github.com/dotcloud/docker/sysinit.SysInit()
        /go/src/github.com/dotcloud/docker/sysinit/sysinit.go:59 +0x564
main.main()
        /go/src/github.com/dotcloud/docker/docker/docker.go:37 +0x67
```

To reproduce, pass empty environment via the API, eg:

```Go
package main

import (
	"fmt"
	docker "github.com/fsouza/go-dockerclient"
	"io"
	"io/ioutil"
)

func main() {
	fmt.Println("begin repro")
	config := &docker.Config{
		Env:   []string{"", ""},
		Image: "ubuntu",
		Cmd:   []string{"echo Testing"},
	}

	endpoint := "unix:///var/run/docker.sock"
	d, err := docker.NewClient(endpoint)
	if err != nil {
		fmt.Printf("ERR newclient: %v", err)
		return
	}

	createOpts := docker.CreateContainerOptions{
		Name: "repro", Config: config}
	c, err := d.CreateContainer(createOpts)
	if err != nil {
		fmt.Printf("ERR createcontainer: %v", err)
		return
	}

	d.StartContainer(c.ID, &docker.HostConfig{})

	r, w := io.Pipe()

	go func() {
		fmt.Println("Reading...")
		data, _ := ioutil.ReadAll(r)
		fmt.Println(string(data))
		fmt.Println("all done.")
	}()

	err = d.AttachToContainer(
		docker.AttachToContainerOptions{
			Container:    c.ID,
			Logs:         true,
			Stream:       true,
			Stdout:       true,
			Stderr:       true,
			OutputStream: w,
			ErrorStream:  w,
		})
	if err != nil {
		fmt.Printf("ERR attachcontainer: %v", err)
		return
	}
	w.Close()

}
```