Fix for fix_fields in derived tables with UNION's ....

mysql-test/r/derived.result

@@ -125,3 +125,7 @@ a	a
 1	NULL
 SELECT * 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'
+SELECT 1 as a FROM (SELECT 1 UNION SELECT a) b;
+Unknown column 'a' in 'field list'

mysql-test/t/derived.test

@@ -45,3 +45,7 @@ SELECT * FROM (SELECT (SELECT * FROM (SELECT 1 as a) as a )) as b;
 select * from (select 1 as a) b  left join (select 2 as a) c using(a);
 --error 1054
 SELECT * FROM (SELECT 1 UNION SELECT a) b;
+--error 1054
+SELECT 1 as a FROM (SELECT a UNION SELECT 1) b;
+--error 1054
+SELECT 1 as a FROM (SELECT 1 UNION SELECT a) b;

sql/item.cc

@@ -573,7 +573,7 @@ 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())