Fix bug with Guild.by_category not showing some channels

If categories come after a child channel in the dictionary mapping for
whatever reason, then the previous code would remove it from the
mapping and set it to []. This commit aims to fix this.

discord/guild.py

@@ -415,7 +415,7 @@ class Guild(Hashable):
         grouped = {}
         for channel in self._channels.values():
             if isinstance(channel, CategoryChannel):
-                grouped[channel.id] = []
+                grouped.setdefault(channel.id, [])
                 continue
 
             try: