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.