Inserting into a temporary table panics after the database is dropped

[djshow832]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql> create database tmpd;
Query OK, 0 rows affected (0.11 sec)

mysql> create temporary table tmpd.t(id int);
Query OK, 0 rows affected (0.01 sec)

mysql> drop database tmpd;
Query OK, 0 rows affected (0.20 sec)

mysql> insert into tmpd.t values(1);
ERROR 1105 (HY000): runtime error: invalid memory address or nil pointer dereference

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

mysql> create database tmpd;
Query OK, 1 row affected (0.03 sec)

mysql> create temporary table tmpd.t(id int);
Query OK, 0 rows affected (0.01 sec)

mysql> drop database tmpd;
Query OK, 0 rows affected (0.07 sec)

mysql> insert into tmpd.t values(1);
Query OK, 1 row affected (0.01 sec)

If inserting cannot succeed, at least it should not panic.

3. What did you see instead (Required)

mysql> insert into tmpd.t values(1);
ERROR 1105 (HY000): runtime error: invalid memory address or nil pointer dereference

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

<!-- Paste the output of SELECT tidb_version() -->

Release Version: None
Edition: Community
Git Commit Hash: None
Git Branch: None
UTC Build Time: None
GoVersion: go1.16.4
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[github-actions]

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