Fix rare crash in FrameTransportImpl::onNext()
Fix rare crash in FrameTransportImpl::onNext()

Summary:
We were depending on the frame processor not having been terminated.  Seems like
in practice it is possible to call FrameTransportImpl::close() first, and then
FrameTransportImpl::onNext() afterwards.

Make onNext() drop any frames it receives.

Reviewed By: phoad

Differential Revision: D8391677

fbshipit-source-id: a901d784971ac84753383035a5c5bcc2bb1039fe

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