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
https://github.com/crossbeam-rs/crossbeam/pull/1187

Signed-off-by: Petros Angelatos <petrosagg@gmail.com>