fix for one of the bugs spotted by #1274

sql/sql_select.cc:
  back to the bug #1274:
  the following query
  EXPLAIN SELECT SQL_CALC_FOUND_ROWS race_name 
  FROM races WHERE race_name LIKE '%Madison%' ORDER BY race_date DESC LIMIT 0,100
  
  +-------+------+---------------+------+---------+------+--------+-----------------------------+
  | table | type | possible_keys | key  | key_len | ref  | rows   | Extra                       |
  +-------+------+---------------+------+---------+------+--------+-----------------------------+
  | races | ALL  | NULL          | NULL |    NULL | NULL | 505821 | Using where; Using filesort |
  +-------+------+---------------+------+---------+------+--------+-----------------------------+
  
  The query returns no rows.
  
  There are two problems with it: 
  - wrong access plan is chosed  (sequential index scan in reverse order,
  which is VERY SLOW in case of MyISAM table + packed keys)
  It's wrong, because it doesn't take into account that SQL_CALC_FOUND_ROWS 
  is present, in other words, is based on assumtion that LIMIT
  clause decrease number of rows to access significantly, which is
  not true as all rows are accessed.
  - the access plan is not shown in the EXPLAIN (bug #1560). I'm not
  fixing it here

sql/sql_select.cc

@@ -968,10 +968,20 @@ mysql_select(THD *thd,TABLE_LIST *tables,List<Item> &fields,COND *conds,
 	}
       }
     }
+    /*
+      Here we sort rows for ORDER BY/GROUP BY clause, if the optimiser
+      chose FILESORT to be faster than INDEX SCAN or there is no 
+      suitable index present.
+      Note, that create_sort_index calls test_if_skip_sort_order and may
+      finally replace sorting with index scan if there is a LIMIT clause in
+      the query. XXX: it's never shown in EXPLAIN!
+      OPTION_FOUND_ROWS supersedes LIMIT and is taken into account.
+    */
     if (create_sort_index(&join.join_tab[join.const_tables],
 			  group ? group : order,
 			  select_limit, 
-			  thd->select_limit))
+                          join.select_options & OPTION_FOUND_ROWS ?
+                          HA_POS_ERROR : thd->select_limit))
       goto err;
   }
   join.having=having;				// Actually a parameter