fix example package

[Syuparn]

close #1357

This fixes runtime panic raised by example app in /_example.

I checked SQL client can obtain response in my local machine.

~/go-mysql-server/_example$ go build
~/go-mysql-server/_example$ ./_example

$ mysql --host=127.0.0.1 --port=3306 --database=mydb -u root
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 1
Server version: 5.7.9-Vitess

Copyright (c) 2000, 2022, Oracle and/or its affiliates.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> select * from mytable;
+----------+-------------------+-------------------------------+---------------------+
| name     | email             | phone_numbers                 | created_at          |
+----------+-------------------+-------------------------------+---------------------+
| Evil Bob | evilbob@gmail.com | ["555-666-555","666-666-666"] | 2018-04-18 09:41:13 |
| Jane Doe | jane@doe.com      | []                            | 2018-04-18 09:41:13 |
| John Doe | john@doe.com      | ["555-555-555"]               | 2018-04-18 09:41:13 |
| John Doe | johnalt@doe.com   | []                            | 2018-04-18 09:41:13 |
+----------+-------------------+-------------------------------+---------------------+
4 rows in set (0.00 sec)

[zachmu]

Daylon, can you figure out what the right answer is here for the example script? I'm a little worried that there's too much setup required to get the in-memory server up and running out of the box (things like requiring the root user). There should be sane defaults for these things.

[Hydrocharged]

@zachmu I'm approving this for now. I've expanded on this greatly, so I'll make another PR. Thank you @Syuparn for bringing this to our attention!