WithLease: always return context and done fn

We should never return a nil context because of the way this function is
typically used... e.g.

```
  ctx, done, err := containerd.WithLease(ctx)
```

If there is an error `ctx` will be nil and any error handling may cause
an NPE if it tries to use `ctx`.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

cmd/ctr/commands/images/unmount.go

@@ -1,16 +1,3 @@
-package images
-
-import (
-	"fmt"
-
-	"github.com/containerd/containerd/cmd/ctr/commands"
-	"github.com/containerd/containerd/errdefs"
-	"github.com/containerd/containerd/leases"
-	"github.com/containerd/containerd/mount"
-	"github.com/pkg/errors"
-	"github.com/urfave/cli"
-)
-
 /*
    Copyright The containerd Authors.
 
@@ -27,6 +14,19 @@ import (
    limitations under the License.
 */
 
+package images
+
+import (
+	"fmt"
+
+	"github.com/containerd/containerd/cmd/ctr/commands"
+	"github.com/containerd/containerd/errdefs"
+	"github.com/containerd/containerd/leases"
+	"github.com/containerd/containerd/mount"
+	"github.com/pkg/errors"
+	"github.com/urfave/cli"
+)
+
 var unmountCommand = cli.Command{
 	Name:        "unmount",
 	Usage:       "unmount the image from the target",