MDEV-34518 Initialise THD::max_tmp_space_used

Author: mariadb-YuchenPei

sql/sql_class.h

@@ -3867,7 +3867,7 @@ class THD: public THD_count, /* this must be first */
   ulonglong  tmp_tables_size;
   ulonglong  bytes_sent_old;
   ulonglong  affected_rows;                     /* Number of changed rows */
-  ulonglong  max_tmp_space_used;
+  ulonglong  max_tmp_space_used= 0;
 
   Opt_trace_context opt_trace;
   pthread_t  real_id;                           /* For debugging */