Item_sum_max/Item_sum_min incorrectly set null_value flag and
attempt to get result in parent functions leads to crash.
This happens due to double evaluation of the function argumet.
First evaluation happens in the comparator and second one
happens in Item_cache::cache_value().
The fix is to introduce new Item_cache object which
holds result of the argument and use this cached value
as an argument of the comparator.

mysql-test/r/func_group.result:
  test case
mysql-test/t/func_group.test:
  test case
sql/item.cc:
  added assertion that ether we have some result or result is NULL.
sql/item_sum.cc:
  introduce new Item_cache object which
  holds result of the argument and use this cached value
  as an argument of the comparator.
sql/item_sum.h:
  introduce new Item_cache object which
  holds result of the argument and use this cached value
  as an argument of the comparator.