Fix regression with references being overwritten from the cache.

A proper fix for this would be to just request presence information
as well since the chunk request would probably be more up to date than
the current cache. However this delta requires a little bit more work
and will be done later.

discord/state.py

@@ -842,7 +842,9 @@ class ConnectionState:
         log.info('Processed a chunk for %s members in guild ID %s.', len(members), guild_id)
         if self._cache_members:
             for member in members:
-                guild._add_member(member)
+                existing = guild.get_member(member.id)
+                if existing is None or existing.joined_at is None:
+                    guild._add_member(member)
 
         self.process_listeners(ListenerType.chunk, guild, len(members))
         names = [x.name.lower() for x in members]