Avoid deadlock in unpacker.

Signed-off-by: Lantao Liu <lantaol@google.com>

pull.go

@@ -70,6 +70,11 @@ func (c *Client) Pull(ctx context.Context, ref string, opts ...RemoteOpt) (_ Ima
 		}
 		unpackWrapper, eg := u.handlerWrapper(ctx, &unpacks)
 		defer func() {
+			if retErr != nil {
+				// Forcibly stop the unpacker if there is
+				// an error.
+				eg.Cancel()
+			}
 			if err := eg.Wait(); err != nil {
 				if retErr == nil {
 					retErr = errors.Wrap(err, "unpack")