assertion of (mem_root->flags & 4) == 0 fails in 2nd ps execution with partition pruning

--source include/have_partition.inc

# 2nd ps mem leak; partition pruning

set @var1=5;

set @var2=4;

create table t1 (a int) partition by list(a) (

  partition p0 values in (null,1,2),

  partition p1 values in (3,4)

);

create table t2 (a int);

insert into t1 values (1),(2),(3),(4);

insert into t2 values (4);

PREPARE stmt FROM 'UPDATE t1 t1 SET a = (SELECT 1 FROM t2 WHERE a = t1.a) where a = ?';

/* 1 */ execute stmt using @var1;

select * from t1;

/* 2 */ execute stmt using @var2;

select * from t1;

deallocate prepare stmt;

drop table t1, t2;