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:
  There's several ways to fix this bug.
  This one looks easy and correct to me