When we create temporary result table for UNION
incorrect max_length for YEAR field is used and
it leads to incorrect field value and incorrect
result string length as YEAR field value calculation
depends on field length.
The fix is to use underlying item max_length for
Item_sum_hybrid::max_length intialization.


mysql-test/r/func_group.result:
  test case
mysql-test/t/func_group.test:
  test case
sql/field.cc:
  added assert
sql/item_sum.cc:
  init Item_sum_hybrid::max_length with 
  use underlying item max_length for
  INT result type.