Safely checks read ring for potentially nil scheduler

[owen-d]

Previously the method signature was (s *Scheduler) SafeReadRing() which checks if s == nil ||. Notably, this can never execute because calling a method on a nil pointer will panic. This fix likely prevents panics when running in microservices mode where the scheduler is not instantiated.

Edit: I stand corrected https://play.golang.org/p/-OfmuRO-Q8f

[slim-bean]

Yeah this kind of blew my mind, but I think this solution feels safer and cleaner so I think we should merge it.