Voice Session invalidated on audio stop but reused in results

[rufferson]

Hi,
_handle_stop_transfer invalidates current session when audio stream stops but then it checks it here to use here while sending results. Probably emulator never sends stop, but IRL it matters.
Trivial fix, don't think worth making fork/PR stuff.

[rufferson]

Oh well it sends stop however reference implementation (transcription_server) has send_result executed from stop callback so it kinda works. But again, if the calls are executed asynchronously and callback does not wait - session gets invalidated prior to response.