panics when set tidb_enable_prepared_plan_cache back to 0

[lcwangchao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

> create table t(a int);
> set @@tidb_enable_prepared_plan_cache=1;
> prepare s from "select * from t";
> set @@tidb_enable_prepared_plan_cache=0;
> execute s;

2. What did you expect to see? (Required)

execute should success

3. What did you see instead (Required)

TiDB root@127.0.0.1:test> execute s;
(1105, 'runtime error: invalid memory address or nil pointer dereference')

4. What is your TiDB version? (Required)

TiDB root@127.0.0.1:test> select tidb_version();
+-----------------------------------------------------------+
| tidb_version()                                            |
+-----------------------------------------------------------+
| Release Version: v7.6.0-alpha                             |
| Edition: Community                                        |
| Git Commit Hash: e62699cb26d6c6b98a52e6e998298dd86c7cdcb7 |
| Git Branch: heads/refs/tags/v7.6.0-alpha                  |
| UTC Build Time: 2023-12-10 14:24:33                       |
| GoVersion: go1.21.5                                       |
| Race Enabled: false                                       |
| Check Table Before Drop: false                            |
| Store: tikv                                               |
+-----------------------------------------------------------+
1 row in set
Time: 0.002s