mariadb_dyncol_column_count returns error when passing an empty dynamic column

static int mdev_4995(MYSQL *mysql)

{

  DYNAMIC_COLUMN dyncol;

  uint column_count= 5;

  mariadb_dyncol_init(&dyncol); /* see patch for MDEV-4993 */

  rc= mariadb_dyncol_column_count(&dyncol,&column_count);

  FAIL_IF(rc < 0, "Error occured");

  FAIL_IF(column_count != 0, "expected column_count=0");

  return OK;

}