Make sure playlist is initialized properly

This stops an error when ocarina is run with an empty library.  Because
the library is empty, the playlist was never being initialized.

libsaria/sources/playlist.py

@@ -6,8 +6,8 @@ import libsaria
 from libsaria.sources import library
 call = libsaria.event.call
 
-song_list = None
-song_set  = None
+song_list = []
+song_set  = set(song_list)
 
 filtered   = False
 visible    = None