added FROM DUAL for derived tables

mysql-test/r/derived.result

+select * from (select 2 from DUAL) b;
+2
+2
 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;

mysql-test/t/derived.test

+select * from (select 2 from DUAL) b;
 -- error 1054
 SELECT 1 as a FROM (SELECT 1 UNION SELECT a) b;
 -- error 1054

sql/sql_yacc.yy

@@ -997,6 +997,7 @@ merge_insert_types:
 
 opt_select_from:
 	opt_limit_clause {}
+	| FROM DUAL_SYM {}
 	| select_from select_lock_type;
 
 udf_func_type: