Only reset the cached feed when the response actually contains headlines

Some feeds, such as those from sourceforge.net, will sometimes show an error
page rather than a feed. In this case the feed cache used to be cleared for
that feed, causing all 'old' headlines to be flooded to the channel as soon
as the feed came back online.

This patch hopefully fixes that by only resetting the cache when the returned
page actually contains headlines.

Signed-off-by: James McCoy <jamessan@users.sourceforge.net>