The Item_func_rollup_const class is used for wrapping constants to avoid
wrong result for ROLLUP queries with DISTINCT and a constant in the select
list. This class is also used to wrap up a NULL constant but its null_value
wasn't set accordingly. This led to a server crash.

Now the null_value of an object of the Item_func_rollup_const class is set
by its fix_length_and_dec member function.


mysql-test/t/olap.test:
  Added a test case for the bug#31095: Unexpected NULL constant caused server crash.
mysql-test/r/olap.result:
  Added a test case for the bug#31095: Unexpected NULL constant caused server crash.
sql/item_func.h:
  Bug#31095: Unexpected NULL constant caused server crash. 
  Now the null_value of an object of the Item_func_rollup_const class is set
  by its fix_length_and_dec member function.