honour the Ad flag in newsfeeds

http://inn.eyrie.org/viewcvs/branches/2.4/innd/art.c?r1=7748&r2=7936&pathrev=7936&view=patch

--- a/innd/art.c
+++ b/innd/art.c
@@ -1682,7 +1682,7 @@ ARTpropagate(Data, hops, hopcount, list)
 	 && !DISTwantany(sp->Distributions, list))
 	    /* Not in the site's desired list of distributions. */
 	    continue;
-	if (sp->DistRequired && list == NULL)
+	if (sp->DistRequired && (list == NULL || *list == NULL))
 	    /* Site requires Distribution header and there isn't one. */
 	    continue;
 
