A fix for a bug in fix_fields in case like this:

select .. UNION select some_column;

This is exhibited in sub-selects and derived tables.

mysql-test/r/derived.result

@@ -123,3 +123,5 @@ 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);
 a	a
 1	NULL
+SELECT * FROM (SELECT 1 UNION SELECT a) b;
+Unknown column 'a' in 'field list'

mysql-test/t/derived.test

@@ -43,3 +43,5 @@ explain select count(*) from t1 as tt1, (select * from t1) as tt2;
 drop table if exists  t1;
 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;

sql/sql_base.cc

@@ -1735,6 +1735,14 @@ find_field_in_tables(THD *thd, Item_ident *item, TABLE_LIST *tables,
   bool allow_rowid= tables && !tables->next;	// Only one table
   for (; tables ; tables=tables->next)
   {
+    if (!tables->table)
+    {
+      if (report_error)
+	my_printf_error(ER_BAD_FIELD_ERROR,ER(ER_BAD_FIELD_ERROR),MYF(0),
+			item->full_name(),thd->where);
+      return (Field*) not_found_field;
+    }
+
     Field *field=find_field_in_table(thd,tables->table,name,length,
 				     grant_option &&
 				     !thd->master_access, allow_rowid);