Fix bug with Guild.by_category not showing some channels
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.

rapptz/discord.py,Harmon758/discord.py,Rapptz/discord.py,Harmon758/discord.py,khazhyk/discord.py