aggregate: Fix aggregation of posts without a title

aggregate: Fix aggregation of posts without a title

When an aggregated post lacked a title, the code first prepended the
$feed->{dir} to it, and only then checked if it had zero length. So,
that check could never succeed and it was possible to end up with
$page="dir/", and writing to that would of course fail.

(Same problem could also occur when the whole title got sanitized away by the
wiki_file_regexp.)

Fixed by simply checking earlier if $page is empty.

Based on a patch by Alexandre Oliva which got lost in a maze of email
folders all alike for over two years despite him mentioning it to me at
least once in person.