Short sum-up, consider that:

* /var/lib/mysql is mount point
* 'systemctl start mariadb' is executed
    * this runs '/usr/bin/mysql_install_db' in /var/lib/mysql when
      the '[ ! -d /var/lib/mysql/mysql ]'
    * if the mysql_install_db fails, 'rm -rf' is called there

The issue is when user mistakenly mounts the directory wrongly.

E.g. the whole '/var' mountpoint mounted as /var/lib/mysql:

* the '/var/lib/mysql/mysql' does not exist
* the mysql_install_db is called, but it fails (permissions)
* the 'rm -rf' is called, removes recursivelly all files owned by mysql

Clear user error, but we can protect user against disaster.

For more info:
+++ This bug was initially created as a clone of Bug #1335849 +++