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 from wip/hadess/fix-atom-podcasts-crash in 9 minutes and 46 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #212778
build_stable

00:09:46

 
Name Stage Failure
failed
build_stable Test
Installing /builds/GNOME/grilo-plugins/grilo/_build/grilo-pls-0.3.pc to /usr/lib64/pkgconfig
Installing /builds/GNOME/grilo-plugins/grilo/_build/meson-private/grilo-0.3.deps to /usr/share/vala/vapi
Installing /builds/GNOME/grilo-plugins/grilo/_build/meson-private/grilo-net-0.3.deps to /usr/share/vala/vapi
Running custom install script '/usr/bin/meson --internal gettext install --subdir=po --localedir=share/locale --pkgname=grilo'
Running custom install script '/usr/bin/meson --internal gtkdoc --sourcedir=/builds/GNOME/grilo-plugins/grilo --builddir=/builds/GNOME/grilo-plugins/grilo/_build --subdir=doc/grilo --headerdirs=/builds/GNOME/grilo-plugins/grilo/src@@/builds/GNOME/grilo-plugins/grilo/libs --mainfile=grilo-docs.sgml --modulename=grilo --moduleversion= --mode=auto --scanargs=--ignore-headers=grl-sync-priv.h grl-sync-priv.h grl-log-priv.h grl-metadata-key-priv.h grl-registry-priv.h grl-plugin-priv.h grl-operation-priv.h grl-operation-options-priv.h --gobjects-types-file=/builds/GNOME/grilo-plugins/grilo/_build/doc/grilo/grilo.types --content-files=/builds/GNOME/grilo-plugins/grilo/doc/grilo/environment-setup.xml@@/builds/GNOME/grilo-plugins/grilo/doc/grilo/plugins-sources.xml@@/builds/GNOME/grilo-plugins/grilo/doc/grilo/plugins-testing.xml@@/builds/GNOME/grilo-plugins/grilo/doc/grilo/quick-start.xml@@/builds/GNOME/grilo-plugins/grilo/doc/grilo/writing-apps.xml@@/builds/GNOME/grilo-plugins/grilo/doc/grilo/overview.xml@@/builds/GNOME/grilo-plugins/grilo/examples/configuring-plugins.c@@/builds/GNOME/grilo-plugins/grilo/examples/browsing.c@@/builds/GNOME/grilo-plugins/grilo/examples/searching.c@@/builds/GNOME/grilo-plugins/grilo/examples/multivalues.c@@/builds/GNOME/grilo-plugins/grilo/examples/efficient-metadata-resolution.c --cc=cc --ld=cc --cflags=-I/builds/GNOME/grilo-plugins/grilo/_build/src/. -I/builds/GNOME/grilo-plugins/grilo/src/. -I/builds/GNOME/grilo-plugins/grilo/_build/src/data -I/builds/GNOME/grilo-plugins/grilo/src/data -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -pthread -I/usr/include/libmount -I/usr/include/blkid -I/usr/include/uuid -I/usr/include/libxml2 --ldflags=-L/builds/GNOME/grilo-plugins/grilo/_build/src -Wl,-rpath,/builds/GNOME/grilo-plugins/grilo/_build/src -lgrilo-0.3 -lglib-2.0 -lgobject-2.0 -pthread -lgmodule-2.0 -lgio-2.0 -lxml2'
FAILED: meson-install
/usr/bin/meson install --no-rebuild
ninja: build stopped: subcommand failed.
ERROR: Job failed: exit code 1