[bugfix] handle stale presences correctly

It's possible for Discord to send us presences for users which do not
actually exist in the Guild (remember; eventually consistent), so we
should throw these away when we get them.

disco/state.py

@@ -188,6 +188,7 @@ class State(object):
                 self.users[member.user.id] = member.user
 
         for presence in event.presences:
+            if presence.user.id in self.users:
                 self.users[presence.user.id].presence = presence
 
         for voice_state in six.itervalues(event.guild.voice_states):