Goto current song return early

We should return early if there is no currently playing song in the
list.

ocarina/source.py

@@ -91,6 +91,8 @@ class ListView(gtk.TreeView):
 
 	def goto(self):
 		id = sources.cur_lib_id
+		if id == -1:
+			return
 		for index, row in enumerate(self.filter_model):
 			if row[0] == id:
 				path = index - 10