Fix #5121: Check if chat messages start with 'name: '

[kimden]

I suppose it's fine to check it naively, as we don't have many players inside one peer, and as the number of messages per peer can already be limited in the server config (and is not big in general).

If there will be a need in many similar checks, I can make a separate data structure for that.

You can test it against the client modified to reproduce this issue.

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