A typo in the Item_cache_datetime::val_str caused an assertion to fail on the
maximum time value.


mysql-test/r/func_group.result:
  A test case for the bug#55648.
mysql-test/t/func_group.test:
  A test case for the bug#55648.
sql/item.cc:
  Bug#55648: Server crash on MIX/MAX on maximum time value
  Corrected assertion.