Rollup merge of #139553 - petrosagg:channel-double-free, r=RalfJung,tgross35

sync::mpsc: prevent double free on `Drop`

This PR is fixing a regression introduced by #121646 that can lead to a double free when dropping the channel.

The details of the bug can be found in the corresponding crossbeam PR crossbeam-rs/crossbeam#1187

Author: matthiaskrgr