Wrong behavior about max prepared stmt

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
ref #39736
```
create table t1
(
a int primary key,
b char(10)
);

prepare stmt4 from 'update t1 set a=? where b=?';
set @arg1=55, @arg2='five';
execute stmt4 using @arg1, @arg2;

prepare stmt4 from 'create table t2 (a int)';
execute stmt4;
deallocate prepare stmt4;

set global max_prepared_stmt_count=1;
prepare stmt from "select 1";
```

### 2. What did you expect to see? (Required)
```
mysql> create table t1
-> (
-> a int primary key,
-> b char(10)
-> );
Query OK, 0 rows affected (0.16 sec)

mysql> prepare stmt4 from 'update t1 set a=? where b=?';
Query OK, 0 rows affected (0.01 sec)
Statement prepared

mysql> set @arg1=55, @arg2='five';
Query OK, 0 rows affected (0.01 sec)

mysql> execute stmt4 using @arg1, @arg2;
Query OK, 0 rows affected (0.01 sec)
Rows matched: 0 Changed: 0 Warnings: 0

mysql> prepare stmt4 from 'create table t2 (a int)';
Query OK, 0 rows affected (0.01 sec)
Statement prepared

mysql> execute stmt4;
Query OK, 0 rows affected (0.02 sec)

mysql> deallocate prepare stmt4;
Query OK, 0 rows affected (0.00 sec)

mysql> set global max_prepared_stmt_count=1;
Query OK, 0 rows affected (0.00 sec)

mysql> prepare stmt from "select 1";
Query OK, 0 rows affected (0.00 sec)
Statement prepared
```

### 3. What did you see instead (Required)
```
mysql> create table t1
-> (
-> a int primary key,
-> b char(10)
-> );
Query OK, 0 rows affected (0.07 sec)

mysql> prepare stmt4 from 'update t1 set a=? where b=?';
Query OK, 0 rows affected (0.00 sec)

mysql> set @arg1=55, @arg2='five';
Query OK, 0 rows affected (0.00 sec)

mysql> execute stmt4 using @arg1, @arg2;
Query OK, 0 rows affected (0.02 sec)
Rows matched: 0 Changed: 0 Warnings: 0

mysql> prepare stmt4 from 'create table t2 (a int)';
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> execute stmt4;
Query OK, 0 rows affected (0.02 sec)

mysql> deallocate prepare stmt4;
Query OK, 0 rows affected (0.00 sec)

mysql> set global max_prepared_stmt_count=1;
Query OK, 0 rows affected (0.01 sec)

mysql> prepare stmt from "select 1";
ERROR 1461 (42000): Can't create more than maxPreparedStmtCount statements (current value: 1)
```

### 4. What is your TiDB version? (Required)
#39736