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

@@ -0,0 +1,8 @@
+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;