(Server crash on DO IFNULL(NULL,NULL)
(fixes also "SELECT CAST(IFNULL(NULL,NULL) as DECIMAL)" unreported
 crash)
(new revampled fix with suggestions from Igor)


mysql-test/r/select.result:
  result of test for bug 12841
mysql-test/t/select.test:
  test for bug #12841
  (Server crash on DO IFNULL(NULL,NULL)
sql/item_func.cc:
  don't use the return value of ::str_op() without checking it
  whether checking it for NULL. (fixes bug #12841 as well as
  another not reported bug, but existing one - test case added).
  All other places where ::str_op() is used are safe.