Proposed fix for #674

This crash happens in rather exotic case when we try to run 
SELECT DISTINCT some_func(SUM(some_field)) GROUP BY another_field;
on a table with single row.
Optimizer marks this table as const, sets group=NULL (with remove_const)
thus, create_tmp_table makes mistake collecting columns for temporary table
and then crashes because the field_count gets less than hidden_columns_count.

sql/sql_select.cc

@@ -453,6 +453,9 @@ mysql_select(THD *thd,TABLE_LIST *tables,List<Item> &fields,COND *conds,
     goto err;
 
   thd->proc_info="preparing";
+
+  select_distinct= select_distinct && (join.const_tables != join.tables);
+
   if (result->initialize_tables(&join))
     goto err;
   if (join.const_table_map != join.found_const_table_map &&