BUG#16200555: EMPTY NAME FOR USER VARIABLE IS ALLOWED AND BREAKS STATEMENT BINARY LOGGING

On a previous fix, user variables with zero length name were incorrectly
considered as event corruption, despite that them are allowed by server.

Fix this wrong assumption by allowing again user variables with zero
length on binary log.

sql/log_event.cc

 /*
-   Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.
+   Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -5729,10 +5729,9 @@ User_var_log_event(const char* buf, uint event_len,
   /*
     We don't know yet is_null value, so we must assume that name_len
     may have the bigger value possible, is_null= True and there is no
-    payload for val.
+    payload for val, or even that name_len is 0.
   */
-  if (0 == name_len ||
-      !valid_buffer_range<uint>(name_len, buf_start, name,
+  if (!valid_buffer_range<uint>(name_len, buf_start, name,
                                 event_len - UV_VAL_IS_NULL))
   {
     error= true;