[youtube] make sure live chat playerOffsetMs is positive

[siikamiika]

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• [x] At least skimmed through adding new extractor tutorial and youtube-dl coding conventions sections
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• [x] Bug fix
• [ ] Improvement
• [ ] New extractor
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Description of your pull request and other information

If the /live_chat_replay/get_live_chat_replay endpoint is given a negative value to playerOffsetMs, it will return 400 bad request. It can happen when the first batch of messages is so large that all of them come before 5 seconds into the video. This pull request defaults the parameter to 0 when it's negative.

It's unclear if setting the parameter actually does anything when sequentially fetching all chat, but at least it looks more like what the browser does.