values for too big argument".

Added range checking for from_unixtime() argument, cleaned up code 
a bit.


mysql-test/r/func_time.result:
  Test for bug #6439 "from_unixtime() function returns wrong datetime 
  values for too big argument".
mysql-test/t/func_time.test:
  Test for bug #6439 "from_unixtime() function returns wrong datetime 
  values for too big argument".
sql/item_timefunc.cc:
  Item_func_from_unixtime: 
    Added error range checking for function argument + small code clean up.