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.cc

@@ -1802,6 +1802,7 @@ void TMP_TABLE_PARAM::init()
   group_parts= group_length= group_null_parts= 0;
   quick_group= 1;
   table_charset= 0;
+  precomputed_group_by= 0;
 }