Update LiveAuthClient.cs for NullReferenceException

Avoid the following exception that is occurring in 5.6 of the LiveSDK on Windows Phone 8

Object reference not set to an instance of an object.
   at Microsoft.Live.LiveAuthClient.RefreshToken(Action`1 completionCallback)
   at Microsoft.Live.Operations.ApiOperation.RefreshTokenIfNeeded()
   at Microsoft.Live.Operations.ApiOperation.PrepareRequest()
   at Microsoft.Live.Operations.ApiOperation.OnExecute()
   at Microsoft.Live.Operations.Operation.InternalExecute()
   at Microsoft.Live.Operations.Operation.Execute()
   at Microsoft.Live.LiveConnectClient.ExecuteApiOperation(ApiOperation op, CancellationToken ct)
   at Microsoft.Live.LiveConnectClient.GetAsync(String path, CancellationToken ct)