MDEV-5717: Server crash with insert statement containing DEFAULT into view

Item_default_value::arg can be NULL so walk() should take it into consideration.

sql/item.h

@@ -2890,7 +2890,7 @@ class Item_default_value : public Item_field
 
   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);
   }