Only reduce message_count if it's a thread

[key2peace]

1004

[SQKo]

No, it wont work like this in forum, i was oversimplyfing this code but overlooked normal channels where it'd be always null, that $parent was going to be same as $channel, it wasnt literally mean only parent channels ( i was saving up variables ) https://github.com/discord-php/DiscordPHP/pull/993/commits/a8a273fb56939464d2d589d8bddca33a9770dfbe#diff-32749737b5ed6d242a013418143adc92a0a33f2182a9553d8f50b4f0a8d667c5L44