Created Issue: NullReferenceException in Renci.SshNet.BaseClient.IsConnected() [1485]

Hi,

while using Renci SSH Client successfully in production, I noticed a really rare error that just occurred on my dev machine:

```
4491 - OXE Control Center Gui: 15:08:15.9481613 - [Remote Connector] 10.132.142.192: Detected stop flag, terminating remote connection...
2482 - OXE Control Center Gui: 15:08:16.0824215 - [Remote Connector] 10.132.142.192: Error on connection check!

Source: Renci.SshNet

Message:
System.NullReferenceException: Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt.
bei Renci.SshNet.Session.IsSocketConnected(Boolean& isConnected)
bei Renci.SshNet.Session.get_IsConnected()
bei Renci.SshNet.BaseClient.get_IsConnected()
bei NextiraOne.Processors.RemoteConnector.IsConnected() in C:\Users\nsk\Documents\Visual Studio 2010\Projects\9 - NextiraOne Libraries\NextiraOne.Core\SP_RemoteConnector.cs:Zeile 393.
```

Notice my generic log message prefixes from my wrapper class around Renci.SshNet that tells us what happened here and in what short timeframe this occurs:
My program execution was stopped due to the internal stop flag and immediately after that the connection state was checked (don't think about the use case as I have a complicated multithreaded environment in which this check may be optimized/blocked at this stop situation, but this is a different topic :D )

The excpected result from IsConnected() is false here.

This is not severe, as my connection check treats errors equal to false. But just to improve Renci.SshNet I post this case here. :)

"Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt." means in english kind of "The object reference does not point to an object".

Regards,
Nicolas