some rollup rows (rows with NULLs for grouping attributes) if GROUP BY
list contained constant expressions.

This happened because the results of constant expressions were not put
in the temporary table used for duplicate elimination. In fact a constant
item from the GROUP BY list of a ROLLUP query can be replaced for an
Item_null_result object when a rollup row is produced . 

Now the JOIN::rollup_init function wraps any constant item referenced in
the GROYP BY list of a ROLLUP query into an Item_func object of a special
class that is never detected as constant item. This ensures creation of
fields for such  constant items in temporary tables and guarantees right
results when the result of the rollup operation first has to be written
into a temporary table, e.g. in the cases when duplicate elimination is
required.  


mysql-test/r/olap.result:
  Added a test case for bug #24856.
mysql-test/t/olap.test:
  Added a test case for bug #24856.
sql/item_func.h:
  Fixed bug #24856: the result set of a ROLLUP query with DISTINCT could lack
  some rollup rows (rows with NULLs for grouping attributes) if GROUP BY
  list contained constant expressions.
  
  Itroduced class Item_func_rollup_const derived from Item_func. The object of
  this class are never detected as constant items.
  We use them for wrapping constant items from the GROUP BY list of any ROLLUP
  query. This wrapping allows us to ensure writing constant items into temporary
  tables whenever the result of the ROLLUP operation has to be written into a
  temporary table, e.g. when ROLLUP is used together with DISTINCT in the SELECT
  list.
sql/sql_select.cc:
  Fixed bug #24856: the result set of a ROLLUP query with DISTINCT could lack
  some rollup rows (rows with NULLs for grouping attributes) if GROUP BY
  list contained constant expressions.
  
  Now the JOIN::rollup_init function wraps any constant item referenced in
  the GROYP BY list of a ROLLUP query into an Item_func object of a special
  class that is never detected as constant item. This ensures creation of
  fields for such  constant items in temporary tables and guarantees right
  results when the result of the rollup operation first has to be written
  into a temporary table, e.g. in the cases when duplicate elimination is
  required.