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

Signed-off-by: Petros Angelatos <[email protected]>

Author: petrosagg

Diff for: library/std/src/sync/mpmc/list.rs

@@ -566,7 +566,7 @@ impl<T> Channel<T> {
             // In that case, just wait until it gets initialized.
             while block.is_null() {
                 backoff.spin_heavy();
-                block = self.head.block.load(Ordering::Acquire);
+                block = self.head.block.swap(ptr::null_mut(), Ordering::AcqRel);
             }
         }