[development] feed aggregator - drupal 6 - minor improvement

I found that the feed aggregator would link into my delicious bookmarks rss OK. Then I wanted to link it in to a feed of my most used tags, but the links did not appear correctly. The .module uses the generic link of the feed in if the item link is not present. On this feed, each item had the correct link value in the guid tag. I don't know how standard that is, but Firefox interprets it correctly. So I added a clause to the replacement algorithm, to use guid value (if it is a valid link) otherwise to revert to the feed link. Patch attached fro the aggregator.module file from version 6.1.2