Add check for nil auth callback

I have seen several `EXC_BAD_ACCESS KERN_INVALID_ADDRESS at 0x0000000000000010` in `+[Foursquare2 callAuthorizationCallbackWithError:]` (`Foursquare2.m,`, line 1470). I assume this is caused my multiple concurrent requests and a race condition that causes the callback to be set to `nil` by one thread and then access by another. Guarded against the `nil` callback and added an `NSAssert` for dev builds to fail-fast.