util/fossilize_db: don't use check_files_opened_successfully for foz_prepare
What does this MR do and why?
util/fossilize_db: don't use check_files_opened_successfully for foz_prepare
The check function will free the db or the file if the other is NULL,
but in this function that will result in a double free foz_destroy.
Instead, check that either is NULL, and then goto the error case.