Don't provide IO when it's not set

This makes sure that runc does not get any valid IO for the pipe.  Some
builds and other containers will be stuck if they inspect stdin
expecially and its a pipe but not connected to any user input.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

runtime/v1/linux/proc/io.go

@@ -109,7 +109,6 @@ func copyPipes(ctx context.Context, rio runc.IO, stdin, stdout, stderr string, w
 		i.dest(fw, fr)
 	}
 	if stdin == "" {
-		rio.Stdin().Close()
 		return nil
 	}
 	f, err := fifo.OpenFifo(ctx, stdin, syscall.O_RDONLY|syscall.O_NONBLOCK, 0)