MDEV-19658 UBSAN: runtime error: load of value 2779096485, which is not a valid value for type 'enum_binlog_format'

This is an uninitialized read.

THD::THD: initialize current_stmt_binlog_format member

sql/sql_class.cc

@@ -1,6 +1,6 @@
 /*
    Copyright (c) 2000, 2015, Oracle and/or its affiliates.
-   Copyright (c) 2008, 2018, MariaDB Corporation.
+   Copyright (c) 2008, 2020, 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
@@ -604,6 +604,7 @@ THD::THD(my_thread_id id, bool is_wsrep_applier)
    m_current_stage_key(0),
    in_sub_stmt(0), log_all_errors(0),
    binlog_unsafe_warning_flags(0),
+   current_stmt_binlog_format(BINLOG_FORMAT_MIXED),
    binlog_table_maps(0),
    bulk_param(0),
    table_map_for_update(0),