Introduced in mark_transaction_to_rollback(), part of fix for bug 24989;
fix is to check thd for NULL before using it.


sql/sql_class.cc:
  It is possible that mark_transaction_to_rollback() may be
  called in rare circumstances when thd is NULL (e.g., from
  some calls to convert_error_code_to_mysql()).  Don't use thd
  if it is NULL.