Fix for a bug with empty item list in UNION's

mysql-test/r/union.result

@@ -273,3 +273,5 @@ SELECT @a:=1 UNION SELECT @a:=@a+1;
 @a:=1
 1
 2
+(SELECT 1) UNION (SELECT 2) ORDER BY (SELECT a);
+Unknown column 'a' in 'field list'

mysql-test/t/union.test

@@ -146,3 +146,5 @@ drop table t1,t2;
 (select 1) union (select 2) order by 0;
 
 SELECT @a:=1 UNION SELECT @a:=@a+1;
+--error 1054
+(SELECT 1) UNION (SELECT 2) ORDER BY (SELECT a);

sql/item.cc

@@ -701,9 +701,10 @@ bool Item_field::fix_fields(THD *thd, TABLE_LIST *tables, Item **ref)
 					 (last= sl)->get_table_list(), &where,
 					 0)) != not_found_field)
 	    break;
-	  if ((refer= find_item_in_list(this, sl->item_list, &counter,
-				       REPORT_EXCEPT_NOT_FOUND)) !=
-	     (Item **)not_found_item)
+	  if (sl->item_list.elements &&
+	      ((refer= find_item_in_list(this, sl->item_list, &counter, 
+					 REPORT_EXCEPT_NOT_FOUND)) != 
+	       (Item **)not_found_item))
 	    break;
 	  if (sl->master_unit()->first_select()->linkage ==
 	      DERIVED_TABLE_TYPE)