fix changes/by_releases for releases without recorded changelog

Author: haarg

lib/MetaCPAN/Server/Controller/Changes.pm

@@ -83,6 +83,8 @@ sub by_releases : Path('by_releases') : Args(0) {
     for my $release ( @{ $ret->{releases} } ) {
         my ( $author, $name, $path )
             = @{$release}{qw(author name changes_file)};
+        next
+            unless $path;
         my $source = $c->model('Source')->path( $author, $name, $path )
             or next;