libcontainerd/client: Fix checkpoint not being set

`cp` variable is used later to populate the `info.Checkpoint` field
option used by Task creation.
Previous changes mistakenly changed assignment of the `cp` variable to
declaration of a new variable that's scoped only to the if block.

Restore the old assignment behavior.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

libcontainerd/remote/client.go

@@ -154,7 +154,8 @@ func (c *container) Start(ctx context.Context, checkpointDir string, withStdin b
 	if checkpointDir != "" {
 		// write checkpoint to the content store
 		tar := archive.Diff(ctx, "", checkpointDir)
-		cp, err := c.client.writeContent(ctx, images.MediaTypeContainerd1Checkpoint, checkpointDir, tar)
+		var err error
+		cp, err = c.client.writeContent(ctx, images.MediaTypeContainerd1Checkpoint, checkpointDir, tar)
 		// remove the checkpoint when we're done
 		defer func() {
 			if cp != nil {