Prevent crash when parsing non XML data
Prevent crash when parsing non XML data

As reported at https://forums.cocoaforge.com/viewtopic.php?f=18&t=27218
Vienna may crash when trying to parse a JSON feed…
Eitot/vienna-rss,Eitot/vienna-rss,josh64x2/vienna-rss,barijaona/vienna-rss,ViennaRSS/vienna-rss,josh64x2/vienna-rss,barijaona/vienna-rss,barijaona/vienna-rss,barijaona/vienna-rss,ViennaRSS/vienna-rss,Eitot/vienna-rss,Eitot/vienna-rss,ViennaRSS/vienna-rss,ViennaRSS/vienna-rss,ViennaRSS/vienna-rss,josh64x2/vienna-rss,josh64x2/vienna-rss,barijaona/vienna-rss,josh64x2/vienna-rss