Fix: if calls Close or Dispose many times it'll throws a NullReferenceException

StackExchange.Redis/StackExchange/Redis/ConnectionMultiplexer.ReaderWriter.cs

@@ -19,7 +19,7 @@ partial class ConnectionMultiplexer
 
         partial void OnCloseReaderWriter()
         {
-            if (ownsSocketManager) socketManager.Dispose();
+            if (ownsSocketManager && socketManager != null) socketManager.Dispose();
             socketManager = null;
         }