convert(<invalid time>, datetime) in WHERE caused crash as function
returned (void*)NULL, but did not flag SQL NULL. It does now.


mysql-test/r/type_datetime.result:
  show that convert() to datetime in a WHERE-clause will no longer crash
  the server on invalid input.
mysql-test/t/type_datetime.test:
  show that convert() to datetime in a WHERE-clause will no longer crash
  the server on invalid input.
sql/item.cc:
  When failing to process time values, do not just return (void*)NULL,
  but flag SQL NULL as well so no caller will try to process the 0x0L.
  This makes behaviour for Item::Item::val_decimal_from_date() and
  Item::Item::val_decimal_from_time() identical.