merge 5.1->5.2

sql/item.h

@@ -2980,7 +2980,7 @@ public:
 
   bool walk(Item_processor processor, bool walk_subquery, uchar *args)
   {
-    return arg->walk(processor, walk_subquery, args) ||
+    return (arg && arg->walk(processor, walk_subquery, args)) ||
       (this->*processor)(args);
   }