MDEV-7938 MariaDB Crashes Suddenly while writing binlogs

on disconnect THD must clean user_var_events array before
dropping temporary tables. Otherwise when binlogging a DROP,
it'll access user_var_events, but they were allocated
in the already freed memroot.

mysql-test/suite/binlog/r/temptable_uservar_disconnect-7938.result

+create table t1 (i int);
+create trigger tr after insert on t1 for each row set @b=@a;
+create temporary table tmp like t1;
+insert into t1 values (1);
+select * from t1;
+i
+1
+drop table t1;

mysql-test/suite/binlog/t/temptable_uservar_disconnect-7938.test

+#
+# MDEV-7938 MariaDB Crashes Suddenly while writing binlogs
+#
+--source include/have_log_bin.inc
+--source include/have_binlog_format_mixed.inc
+
+--connect(con1,localhost,root,,)
+create table t1 (i int);
+create trigger tr after insert on t1 for each row set @b=@a;
+
+create temporary table tmp like t1;
+insert into t1 values (1);
+--disconnect con1
+
+--connection default
+select * from t1;
+drop table t1;

sql/sql_class.cc

@@ -1394,6 +1394,7 @@ void THD::cleanup(void)
   mysql_ha_cleanup(this);
   locked_tables_list.unlock_locked_tables(this);
 
+  delete_dynamic(&user_var_events);
   close_temporary_tables(this);
 
   transaction.xid_state.xa_state= XA_NOTR;
@@ -1427,7 +1428,6 @@ void THD::cleanup(void)
   debug_sync_end_thread(this);
 #endif /* defined(ENABLED_DEBUG_SYNC) */
 
-  delete_dynamic(&user_var_events);
   my_hash_free(&user_vars);
   sp_cache_clear(&sp_proc_cache);
   sp_cache_clear(&sp_func_cache);