Fix a compile error (missing return value).

sql/item_func.h:
  Fix a compoile error (missing return value) as proposed by Tomas Ulin.

sql/item_func.h

@@ -348,7 +348,7 @@ class Item_func_div :public Item_num_op
 {
 public:
   Item_func_div(Item *a,Item *b) :Item_num_op(a,b) {}
-  longlong int_op() { DBUG_ASSERT(0); }
+  longlong int_op() { DBUG_ASSERT(0); return 0; }
   double real_op();
   my_decimal *decimal_op(my_decimal *);
   const char *func_name() const { return "/"; }