As get_arg0_date() in the Item_func_last_day::get_date() returns 
0000-00-00 date sometimes, we have to check ltime->month for 0 after the call.


mysql-test/r/func_time.result:
  Fix for bug #23653: Crash if last_day('0000-00-00')
    - test result.
mysql-test/t/func_time.test:
  Fix for bug #23653: Crash if last_day('0000-00-00')
    - test case.
sql/item_timefunc.cc:
  Fix for bug #23653: Crash if last_day('0000-00-00')
    - return error if month is 0.