Fix for BUG#14920 Ordering aggregated result sets corrupts resultset.

The cause of the bug was the use of end_write_group instead of end_write
in the case when ORDER BY required a temporary table, which didn't take
into account the fact that loose index scan already computes the result
of MIN/MAX aggregate functions (and performs grouping).

The solution is to call end_write instead of end_write_group and to add
the MIN/MAX functions to the list of regular functions so that their
values are inserted into the temporary table.

sql/sql_class.h

@@ -1819,11 +1819,18 @@ public:
   uint  convert_blob_length; 
   CHARSET_INFO *table_charset; 
   bool schema_table;
+  /*
+    True if GROUP BY and its aggregate functions are already computed
+    by a table access method (e.g. by loose index scan). In this case
+    query execution should not perform aggregation and should treat
+    aggregate functions as normal functions.
+  */
+  bool precomputed_group_by;
 
   TMP_TABLE_PARAM()
     :copy_field(0), group_parts(0),
      group_length(0), group_null_parts(0), convert_blob_length(0),
-     schema_table(0)
+     schema_table(0), precomputed_group_by(0)
   {}
   ~TMP_TABLE_PARAM()
   {