don't set derived->merged until derived is really irreversibly merged

it was set before big if() that could decide not to merge and
go with materialization.

this fixes a crash in main.view test

sql/sql_derived.cc

@@ -388,7 +388,6 @@ bool mysql_derived_merge(THD *thd, LEX *lex, TABLE_LIST *derived)
    thd->save_prep_leaf_list= TRUE;
 
   arena= thd->activate_stmt_arena_if_needed(&backup);  // For easier test
-  derived->merged= TRUE;
 
   if (!derived->merged_for_insert || 
       (derived->is_multitable() && 
@@ -452,6 +451,7 @@ bool mysql_derived_merge(THD *thd, LEX *lex, TABLE_LIST *derived)
     if (parent_lex->join) 
       parent_lex->join->table_count+= dt_select->join->table_count - 1;
   }
+  derived->merged= TRUE;
   if (derived->get_unit()->prepared)
   {
     Item *expr= derived->on_expr;