Fix bug with member voice state update.

discord/server.py

@@ -95,7 +95,8 @@ class Member(User):
         if old_channel is None and self.voice_channel is not None:
             # we joined a channel
             self.voice_channel.voice_members.append(self)
-        elif old_channel is not None and self.voice_channel is None and self in old_channel:
+        elif old_channel is not None and self.voice_channel is None:
+            if self in old_channel.voice_members:
                 # we left a channel
                 old_channel.voice_members.remove(self)