radiofrance: Don't break when run behind a portal

[lua-library] grl-lua-library.c:351: calling source callback function fail (radiofrance_fetch_cb) '/home/hadess/Projects/gnome-install/share/grilo-plugins/grl-lua-factory/grl-radiofrance.lua:87: attempt to index local 'items' (a nil value)'

https://bugzilla.gnome.org/show_bug.cgi?id=727569