Work around the crash in MDEV-17814

Internal transactions may not have trx->mysql_thd.
But at the same time, trx->duplicates should only hold if
REPLACE or INSERT...ON DUPLICATE KEY UPDATE was executed from SQL.

The flag feels misplaced. A more appropriate place for it would
be row_prebuilt_t or similar.

sql/sql_class.cc

 /*
    Copyright (c) 2000, 2015, Oracle and/or its affiliates.
-   Copyright (c) 2008, 2017, MariaDB Corporation.
+   Copyright (c) 2008, 2018, MariaDB Corporation.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -4525,7 +4525,8 @@ extern "C" int thd_rpl_is_parallel(const MYSQL_THD thd)
 
 extern "C" int thd_rpl_stmt_based(const MYSQL_THD thd)
 {
-  return !thd->is_current_stmt_binlog_format_row() &&
+  return thd &&
+    !thd->is_current_stmt_binlog_format_row() &&
     !thd->is_current_stmt_binlog_disabled();
 }