Merge pull request #426 from u-abramchuk/master

fix NullReferenceException when retriving LastException from ServerEn…

StackExchange.Redis/StackExchange/Redis/ServerEndPoint.cs

@@ -101,7 +101,8 @@ internal Exception LastException
                 //check if subscription endpoint has a better lastexception
                 if (tmp2 != null && tmp2.LastException != null)
                 {
-                    if (!tmp2.LastException.Data["Redis-FailureType"].ToString().Equals(ConnectionFailureType.UnableToConnect.ToString()))
+                    var failureType = tmp2.LastException.Data["Redis-FailureType"];
+                    if (failureType != null && !failureType.ToString().Equals(ConnectionFailureType.UnableToConnect.ToString()))
                     {
                         return tmp2.LastException;
                     }