flickr: Check return of xml*Get() functions
flickr: Check return of xml*Get() functions

That way we make sure the property exists and was properly parsed.

https://bugzilla.gnome.org/show_bug.cgi?id=707643

MikePetullo/grilo-plugins,MikePetullo/grilo-plugins,jasuarez/grilo-plugins,MikePetullo/grilo-plugins,grilofw/grilo-plugins,GNOME/grilo-plugins,jasuarez/grilo-plugins,grilofw/grilo-plugins,GNOME/grilo-plugins