Fix for the NPE with concurrent threads (issue #91)

When two or more concurrent threads are downloading a file and they are generating a new one simultaneously the method *stop* of ***MapPrinter*** object is invoked and it puts the *config* var to **null** so when the ***MapPrinter*** tries to access to *config* var a NPE is thrown. This fix checks if the *config* instance variable is **null** before to use it.
Jmeter test:

![seleccion_067](https://cloud.githubusercontent.com/assets/1272750/4402840/d22bbec8-449b-11e4-888c-e2107c66400d.png)