Fix: PHP undefined notice in caching file storage

see: http://issues.joomla.org/tracker/joomla-cms/2535 & https://github.com/joomla/joomla-cms/issues/2535

#### Steps to reproduce the issue
1. configure caching to be file based in configuration.php
2. use callback as handler 
3. lock the id
4. remove the cache file 
5. unlock the id now   

#### Expected result
If the cache file has beem removed, the unlock should just return true. 

#### Actual result
Get php undefined variable notice: 
Notice: Undefined variable: ret in web/libraries/joomla/cache/storage/file.php on line <i>333</i>

#### System information (as much as possible)
Linux, Joomla 3.2 latest.