Fix panic of "docker stats --format {{.Name}} --all"

This commit fixes panic when execute stats command:

* use --format {{.Name}} with --all when there're exited containers.
* use --format {{.Name}} while stating exited container.

The root cause is when stating an exited container, the result from the
api didn't contain the Name and ID field, which will make format
process panic.

Panic log is like this:

```
panic: runtime error: slice bounds out of range [recovered]
	panic: runtime error: slice bounds out of range

goroutine 1 [running]:
panic(0xb20f80, 0xc420014110)
	/usr/local/go/src/runtime/panic.go:500 +0x1a1
text/template.errRecover(0xc4201773e8)
	/usr/local/go/src/text/template/exec.go:140 +0x2ad
panic(0xb20f80, 0xc420014110)
	/usr/local/go/src/runtime/panic.go:458 +0x243
github.com/docker/docker/cli/command/formatter.(*containerStatsContext).Name(0xc420430160,
0x0, 0x0)
	/go/src/github.com/docker/docker/cli/command/formatter/stats.go:148
+0x86
reflect.Value.call(0xb9a3a0, 0xc420430160, 0x2213, 0xbe3657, 0x4,
0x11bc9f8, 0x0, 0x0, 0x4d75b3, 0x1198940, ...)
	/usr/local/go/src/reflect/value.go:434 +0x5c8
reflect.Value.Call(0xb9a3a0, 0xc420430160, 0x2213, 0x11bc9f8, 0x0, 0x0,
0xc420424028, 0xb, 0xb)
	/usr/local/go/src/reflect/value.go:302 +0xa4
text/template.(*state).evalCall(0xc420177368, 0xb9a3a0, 0xc420430160,
0x16, 0xb9a3a0, 0xc420430160, 0x2213, 0x1178fa0, 0xc4203ea330,
0xc4203de283, ...)
	/usr/local/go/src/text/template/exec.go:658 +0x530
```

Signed-off-by: Zhang Wei <zhangwei555@huawei.com>

cli/command/formatter/stats.go

@@ -149,8 +149,10 @@ func (c *containerStatsContext) Container() string {
 
 func (c *containerStatsContext) Name() string {
 	c.AddHeader(nameHeader)
-	name := c.s.Name[1:]
-	return name
+	if len(c.s.Name) > 1 {
+		return c.s.Name[1:]
+	}
+	return "--"
 }
 
 func (c *containerStatsContext) ID() string {

cli/command/formatter/stats_test.go

@@ -69,6 +69,12 @@ func TestContainerStatsContextWrite(t *testing.T) {
 			`MEM USAGE / LIMIT
 20 B / 20 B
 -- / --
+`,
+		},
+		{
+			Context{Format: "{{.Container}}  {{.ID}}  {{.Name}}"},
+			`container1  abcdef  foo
+container2    --
 `,
 		},
 		{
@@ -83,6 +89,8 @@ container2  --
 		stats := []StatsEntry{
 			{
 				Container:        "container1",
+				ID:               "abcdef",
+				Name:             "/foo",
 				CPUPercentage:    20,
 				Memory:           20,
 				MemoryLimit:      20,