podcast: throw away parsed data if the dialog has been reset
podcast: fix parsed podcast data double free

When we think we're parsing an existing feed, free the parsed data
if it doesn't match up with anything in the feed list.  Otherwise,
create episodes if it's the currently selected feed.  The logic here
was a bit messed up.

https://bugzilla.gnome.org/show_bug.cgi?id=733499