Missing error handling for Mage::getResourceSingleton

This is just a simple flaw of to a missing return value check. This error is common to be done as the Magento core-code is full of these. I normally do not fix these however this is an external extension and the missing error check did introduce a crash on my end with changes to firegento logger `config.xml` in #30.

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That global static class method does error handling not only by throwing
exceptions but also by returning FALSE.

This change fixes the flaw by introducing a type-check on the variable by
returning void on falsely.

How to reproduce:

When that model has been disabled in config.xml, there is no such
cleanLogs() method available as this is a non-object. See #30.