Checking for NULL before calling the val_xxx()
 methods only checks for such arguments that are 
 known to be NULLs at compile time. 
 The arguments that may or may not contain
 NULLs (e.g. function calls and possibly others)
 are not checked at all.
 Fixed by first calling the val_xxx() method and
 then checking for null in SEC_TO_TIME().
 In addition QUARTER() was not returning 0 (as all the 
 val_int() functions do when processing a NULL value).


mysql-test/r/func_time.result:
  Bug #25643: SEC_TO_TIME function problem
   - test case
mysql-test/t/func_time.test:
  Bug #25643: SEC_TO_TIME function problem
   - test case
sql/item_timefunc.cc:
  Bug #25643: SEC_TO_TIME function problem
   - null handling fixed for QUARTER() and SEC_TO_TIME()