c.RWLayer: check for nil before use

Since commit e9b9e4ace2 has landed, there is a chance that
container.RWLayer is nil (due to some half-removed container). Let's
check the pointer before use to avoid any potential nil pointer
dereferences, resulting in a daemon crash.

Note that even without the abovementioned commit, it's better to perform
an extra check (even it's totally redundant) rather than to have a
possibility of a daemon crash. In other words, better be safe than
sorry.

[v2: add a test case for daemon.getInspectData]
[v3: add a check for container.Dead and a special error for the case]

Fixes: e9b9e4ace2
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

daemon/inspect_test.go

@@ -0,0 +1,33 @@
+package daemon // import "github.com/docker/docker/daemon"
+
+import (
+	"testing"
+
+	containertypes "github.com/docker/docker/api/types/container"
+	"github.com/docker/docker/container"
+	"github.com/docker/docker/daemon/config"
+	"github.com/docker/docker/daemon/exec"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGetInspectData(t *testing.T) {
+	c := &container.Container{
+		ID:           "inspect-me",
+		HostConfig:   &containertypes.HostConfig{},
+		State:        container.NewState(),
+		ExecCommands: exec.NewStore(),
+	}
+
+	d := &Daemon{
+		linkIndex:   newLinkIndex(),
+		configStore: &config.Config{},
+	}
+
+	_, err := d.getInspectData(c)
+	assert.Error(t, err)
+
+	c.Dead = true
+	_, err = d.getInspectData(c)
+	assert.NoError(t, err)
+}