fix for bug #15828 after review
  doing val_str now before testing of null value secures the function for null values returned by dynamic functions - the fix before was incomplete andy covered constant null values


mysql-test/t/date_formats.test:
  fix for bug #15828 after review
  doing val_str now before testing of null value secures the function for null values returned by dynamic functions - the fix before was incomplete andy covered constant null values