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.

CHANGELOG

@@ -7,6 +7,10 @@ ikiwiki (3.20180312) UNRELEASED; urgency=medium
   [ Simon McVittie ]
   * Hide popup template content from documentation (Closes: #898836)
 
+  [ Joey Hess ]
+  * aggregate: Fix aggregation of posts without a title.
+    Thanks, Alexandre Oliva
+
  -- Simon McVittie <smcv@debian.org>  Wed, 16 May 2018 13:09:27 +0100
 
 ikiwiki (3.20180311) upstream; urgency=medium

IkiWiki/Plugin/aggregate.pm

@@ -621,11 +621,11 @@ sub add_page (@) {
 		# escape slashes and periods in title so it doesn't specify
 		# directory name or trigger ".." disallowing code.
 		$page=~s!([/.])!"__".ord($1)."__"!eg;
-		$page=$feed->{dir}."/".$page;
-		($page)=$page=~/$config{wiki_file_regexp}/;
 		if (! defined $page || ! length $page) {
 			$page=$feed->{dir}."/item";
 		}
+		$page=$feed->{dir}."/".$page;
+		($page)=$page=~/$config{wiki_file_regexp}/;
 		my $c="";
 		while (exists $IkiWiki::pagecase{lc $page.$c} ||
 		       -e $IkiWiki::Plugin::transient::transientdir."/".htmlfn($page.$c) ||