Prevent crash if onComplete is called on a cancelled subscriber
Prevent crash if onComplete is called on a cancelled subscriber

alexmalyshev/rsocket-cpp,phoad/rsocket-cpp,alexmalyshev/rsocket-cpp,phoad/rsocket-cpp,rsocket/rsocket-cpp,phoad/rsocket-cpp,alexmalyshev/rsocket-cpp,rsocket/rsocket-cpp,rsocket/rsocket-cpp,phoad/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,alexmalyshev/rsocket-cpp,rsocket/rsocket-cpp,phoad/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,ReactiveSocket/reactivesocket-cpp