Fix _ProactorWritePipeTransport._pipe_closed()
Fix _ProactorWritePipeTransport._pipe_closed()

Do nothing if the pipe is already closed. _loop_writing() may call
_force_close() when it gets ConnectionResetError.

overcastcloud/trollius,overcastcloud/trollius,leetreveil/tulip,overcastcloud/trollius,leetreveil/tulip,leetreveil/tulip