Fixes nil string crash

I am not sure if this is a true fix but I was getting a crash in my app that was: [NSURL initFileURLWithPath:]: nil string parameter and I traced it back to the NSURL *fontURL line. Anyway after some logging, it seemed that registerCustomTTF was getting called twice. I am not sure why that is exactly as I am using the latest version of SpriteBuilder but the second time, fontPath was NULL and causing a crash. Anyway this simple check will simply avoid that crash as an immediate fix but hopefully this will bring awareness to this issue and perhaps you know of a better fix or a reason that this crash was happening in the first place.