podcasts: Fix crash trying to parse unsupported format

1. Add an Atom feed to the Podcasts plugin
2. Try to browse that Atom feed
3. RSS parser crashes as it doesn't know how to parse Atom

==8560== Invalid read of size 8
==8560==    at 0x1BBD36DF: parse_feed (grl-podcasts.c:1274)
==8560==    by 0x1BBD36DF: read_feed_cb (grl-podcasts.c:1365)
==8560==    by 0x1BBD1272: read_done_cb (grl-podcasts.c:494)
==8560==    by 0x52F1A69: g_simple_async_result_complete (in /usr/lib64/libgio-2.0.so.0.5800.3)
==8560==    by 0x1BB48D2F: read_async_cb (grl-net-wc.c:619)
==8560==  Address 0x10 is not stack'd, malloc'd or (recently) free'd
1 job for master in 3 minutes and 12 seconds (queued for 1 second)
Status Job ID Name Coverage
passed #212879