`GBPublishedFeedFormatsFromNSString` should initialize `formats`

A trivial little change: `GBPublishedFeedFormatsFromNSString` is not currently initializing the `formats` variable. I'm initializing it to `0`. If you want to initialize it to something else (e.g. I don't know if you're initializing `self.settings.docsetFeedFormats` elsewhere), feel free, but to leave it uninitialized will result in undefined behavior.