PUT returns null, leading to a NullPointerException when debug loggin…

…g is enabled in SocketStore

src/java/voldemort/store/socket/SocketStore.java

@@ -445,7 +445,7 @@ private void invokeCallback(Object o, long requestTime) {
                                      + clientRequestExecutor.getSocketChannel()
                                                             .socket()
                                                             .getLocalPort() + " result: "
-                                     + o.toString());
+                                     + o);
                     }
 
                     callback.requestComplete(o, requestTime);