fixed compatibility issue for "mysql_" plugin with MySQL 5.5/5.6.

I have found two compatibility issues for "mysql_" plugin with MySQL 5.5/5.6.
Would you please review to merge for plugin of "mysql_" to work with MySQL 5.5/5.6 ?

### 1. made a patch for "INDIVIDUAL BUFFER POOL INFO" section

The mysql plugin named "mysql_" have a problem. So It could not work fine with MySQL 5.5 or 5.6.
I have make a patch for "INDIVIDUAL BUFFER POOL INFO" section asskipped to work.

#### all of symlinked mysql_ pluugin gets error like below.
```sh
$ /etc/munin/plugins/mysql_connections
Unknown section: INDIVIDUAL BUFFER POOL INFO at /etc/munin/plugins/mysql_connections line 1098.
```

#### related issue
https://github.com/kjellm/munin-mysql/commit/111293966ccd6e8df6c1347cb0f9a8b4025c0266

## 2. made a patch for "mysql_innodb_insert_buf" to work

The mysql plugin named "mysql_" have a problem. So It could not work fine with MySQL 5.5 or 5.6.
I have make a patch for "mysql_innodb_insert_buf" to work.

#### symlinked from mysql_innodb_insert_buf to mysql_ pluugin gets error below.
```sh
$ /etc/munin/plugins/mysql_innodb_insert_buf
Use of uninitialized value in printf at /etc/munin/plugins/mysql_innodb_insert_buf line 885.
ib_ibuf_inserts.value
Use of uninitialized value in printf at /etc/munin/plugins/mysql_innodb_insert_buf line 885.
ib_ibuf_merged_rec.value
Use of uninitialized value in printf at /etc/munin/plugins/mysql_innodb_insert_buf line 885.
ib_ibuf_merges.value
```

#### related issue
https://github.com/kjellm/munin-mysql/issues/34#issuecomment-1487824