Cleanup usage of DBUG_ASSERTS.

sql/item.cc

@@ -8075,7 +8075,6 @@ bool Item_direct_view_ref::send(Protocol *protocol, String *buffer)
 
 bool Item_direct_view_ref::fix_fields(THD *thd, Item **reference)
 {
-  DBUG_ASSERT(1);
   /* view fild reference must be defined */
   DBUG_ASSERT(*ref);
   /* (*ref)->check_cols() will be made in Item_direct_ref::fix_fields */
@@ -9959,7 +9958,7 @@ void Item_direct_view_ref::update_used_tables()
 
 table_map Item_direct_view_ref::used_tables() const
 {
-  DBUG_ASSERT(null_ref_table);
+  DBUG_ASSERT(fixed);
 
   if (get_depended_from())
     return OUTER_REF_TABLE_BIT;