bpo-35682: Fix _ProactorBasePipeTransport._force_close() (GH-11462)

bpo-32622, bpo-35682: Fix asyncio.ProactorEventLoop.sendfile(): don't
attempt to set the result of an internal future if it's already done.

Fix asyncio _ProactorBasePipeTransport._force_close(): don't set the
result of _empty_waiter if it's already done.