mysql_innodb_insert_buf compatibility problem fixed for MySQL 5.5/5.6

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