Bug fix for BadHexadecimalConversionDetector

Hi, 
I've just noticed a little bug in the BadHexadecimalConversionDetector when I was using the find-sec-bugs in my project. Actually, a nullPointerEx was notified by maven and eclipse (in the log view) when I scanned my project. 
It seemed like "classContext.getMethodGen(m)" return null for any method in any interfaces.

I hope it can help you and thanks for your findbugs plugin.

Patrice CAVEZZAN