fixed derived table visibility scope for derived tables with union

mysql-test/r/derived.result:
  test of error messages
mysql-test/t/derived.test:
   SELECT 1 as a FROM (SELECT 1 UNION SELECT a) b;

mysql-test/r/derived.result

+SELECT 1 as a FROM (SELECT 1 UNION SELECT a) b;
+Unknown column 'a' in 'field list'
+SELECT 1 as a FROM (SELECT a UNION SELECT 1) b;
+Unknown column 'a' in 'field list'
 drop table if exists t1,t2,t3;
 CREATE TABLE t1 (a int not null, b char (10) not null);
 insert into t1 values(1,'a'),(2,'b'),(3,'c'),(3,'c');

mysql-test/t/derived.test

+-- error 1054
+SELECT 1 as a FROM (SELECT 1 UNION SELECT a) b;
+-- error 1054
+SELECT 1 as a FROM (SELECT a UNION SELECT 1) b;
 drop table if exists t1,t2,t3;
 CREATE TABLE t1 (a int not null, b char (10) not null);
 insert into t1 values(1,'a'),(2,'b'),(3,'c'),(3,'c');

sql/item.cc

@@ -573,7 +573,8 @@ bool Item_field::fix_fields(THD *thd, TABLE_LIST *tables, Item **ref)
       Item **refer= (Item **)not_found_item;
       // Prevent using outer fields in subselects, that is not supported now
       SELECT_LEX *cursel=(SELECT_LEX *) thd->lex.current_select;
-      if (cursel->linkage != DERIVED_TABLE_TYPE)
+      if (cursel->master_unit()->first_select()->linkage !=
+	  DERIVED_TABLE_TYPE)
 	for (SELECT_LEX *sl=cursel->outer_select();
 	     sl;
 	     sl= sl->outer_select())
@@ -586,7 +587,8 @@ bool Item_field::fix_fields(THD *thd, TABLE_LIST *tables, Item **ref)
 				       REPORT_EXCEPT_NOT_FOUND)) !=
 	     (Item **)not_found_item)
 	    break;
-	  if (sl->linkage == DERIVED_TABLE_TYPE)
+	  if (sl->master_unit()->first_select()->linkage ==
+	      DERIVED_TABLE_TYPE)
 	    break; // do not look over derived table
 	}
       if (!tmp)
@@ -1024,7 +1026,8 @@ bool Item_ref::fix_fields(THD *thd,TABLE_LIST *tables, Item **reference)
     if ((ref= find_item_in_list(this, 
 				*(thd->lex.current_select->get_item_list()),
 				((sl && 
-				  thd->lex.current_select->linkage !=
+				  thd->lex.current_select->master_unit()->
+				  first_select()->linkage !=
 				  DERIVED_TABLE_TYPE) ? 
 				  REPORT_EXCEPT_NOT_FOUND :
 				  REPORT_ALL_ERRORS))) ==
@@ -1050,7 +1053,8 @@ bool Item_ref::fix_fields(THD *thd,TABLE_LIST *tables, Item **reference)
 	if ((tmp= find_field_in_tables(thd, this,
 				       sl->get_table_list(),
 				       0)) != not_found_field);
-	if (sl->linkage == DERIVED_TABLE_TYPE)
+	if (sl->master_unit()->first_select()->linkage ==
+	    DERIVED_TABLE_TYPE)
 	  break; // do not look over derived table
       }