Simplify logic for `case .error, .completed`
Simplify logic for `case .error, .completed`

I thought that instead of returning on `!AtomicCompareAndSwap(0, 1, &_isStopped)`
it would read easier to just switch the logic of the if statement and call `onCore(event)` 
in the body if the condition passes.
kzaher/RxSwift,kzaher/RxSwift,ReactiveX/RxSwift,kzaher/RxSwift,kzaher/RxSwift,ReactiveX/RxSwift,ReactiveX/RxSwift