temporary fix for problem caused while checking existence of communication port.

The communication port cannot be shared with others in Windows(it means we cannot use CreateFile to check exactly), and _access() or access()(deprecated) in Windows is not work as it of linux for checking existence of communication port. 

However _access() says "Access is denied" or "Invalid parameter" when a communication port is opened by someone. Using this behavior, I applied 'Band-Aid'.

I think it had better be replaced by new scheme for Windows; To call serial.open(...) directly, and detect disconnection using WaitCommEvent in another thread owned by Serial object. Next we use this connection flag.