add mpsc memory leak to trophy case

Author: RalfJung

README.md

@@ -588,6 +588,7 @@ Definite bugs found:
 * [Dropping with unaligned pointers in `vec::IntoIter`](https://github.com/rust-lang/rust/pull/106084)
 * [Deallocating with the wrong layout in new specializations for in-place `Iterator::collect`](https://github.com/rust-lang/rust/pull/118460)
 * [Incorrect offset computation for highly-aligned types in `portable-atomic-util`](https://github.com/taiki-e/portable-atomic/pull/138)
+* [Occasional memory leak in `std::mpsc` channels](https://github.com/rust-lang/rust/issues/121582) (original code in [crossbeam](https://github.com/crossbeam-rs/crossbeam/pull/1084))
 
 Violations of [Stacked Borrows] found that are likely bugs (but Stacked Borrows is currently just an experiment):