Curios Sync Fix

Fixes the “Failed decoding custom payload curios:sync_stack” error that causes you to be disconnected, even from integrated server (i.e. local play, no server).

github issue: https://github.com/TheIllusiveC4/Curios/issues/635

Pull Request was provided for the 26.x branch, but fix tested for 1.21.1: https://github.com/TheIllusiveC4/Curios/pull/643

Update: The PR was merged into the 26.x branch (yay!) and TheIllusiveC4 said they would look into backporting, including to 1.21.1 (which is my version of interest; also yay!).