fix SB violation in LeakedBuffers

```
error: Undefined Behavior: trying to retag from <335802> for Unique permission at alloc141485[0x0], but that tag does not exist in the borrow stack for this location
   --> tokio/tests/support/leaked_buffers.rs:24:9
    |
24  |         slice
    |         ^^^^^
    |         |
    |         trying to retag from <335802> for Unique permission at alloc141485[0x0], but that tag does not exist in the borrow stack for this location
    |         this error occurs as part of retag at alloc141485[0x0..0xa]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <335802> was created by a Unique retag at offsets [0x0..0xa]
   --> tokio/tests/support/leaked_buffers.rs:22:21
    |
22  |         let slice = std::slice::from_raw_parts_mut(new_mem.as_mut_ptr(), new_mem.len());
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: <335802> was later invalidated at offsets [0x0..0xa] by a Unique retag
   --> tokio/tests/support/leaked_buffers.rs:23:31
    |
23  |         self.leaked_vecs.push(new_mem);
    |                               ^^^^^^^
    = note: BACKTRACE (of the first span):
    = note: inside `support::leaked_buffers::LeakedBuffers::create::<'_>` at tokio/tests/support/leaked_buffers.rs:24:9: 24:14
note: inside `<BadAsyncRead as tokio::io::AsyncRead>::poll_read`
   --> tokio/tests/io_read.rs:63:38
    |
63  |         *buf = ReadBuf::new(unsafe { self.leaked_buffers.create(buf.capacity()) });
    |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `<&mut BadAsyncRead as tokio::io::AsyncRead>::poll_read`
   --> /home/runner/work/tokio/tokio/tokio/src/io/async_read.rs:77:5
    |
77  |     deref_async_read!();
    |     ^^^^^^^^^^^^^^^^^^^
note: inside `<tokio::io::util::read_buf::ReadBuf<'_, BadAsyncRead, std::vec::Vec<u8>> as std::future::Future>::poll`
   --> /home/runner/work/tokio/tokio/tokio/src/io/util/read_buf.rs:57:20
    |
57  |             ready!(Pin::new(me.reader).poll_read(cx, &mut buf)?);
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside closure
   --> tokio/tests/io_read.rs:74:43
    |
74  |     BadAsyncRead::new().read_buf(&mut buf).await.unwrap();
    |                                           ^^^^^^
    = note: inside `<std::pin::Pin<&mut dyn std::future::Future<Output = ()>> as std::future::Future>::poll` at /home/runner/.rustup/toolchains/nightly-2022-12-27-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/future/future.rs:124:9: 124:61
    = note: inside `<std::pin::Pin<&mut std::pin::Pin<&mut dyn std::future::Future<Output = ()>>> as std::future::Future>::poll` at /home/runner/.rustup/toolchains/nightly-2022-12-27-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/future/future.rs:124:9: 124:61
note: inside closure
   --> /home/runner/work/tokio/tokio/tokio/src/runtime/scheduler/current_thread.rs:541:57
    |
541 |                         crate::runtime::coop::budget(|| future.as_mut().poll(&mut cx))
    |                                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `tokio::runtime::coop::with_budget::<std::task::Poll<()>, [closure@tokio::runtime::scheduler::current_thread::CoreGuard<'_>::block_on<std::pin::Pin<&mut std::pin::Pin<&mut dyn std::future::Future<Output = ()>>>>::{closure#0}::{closure#0}::{closure#0}]>`
   --> /home/runner/work/tokio/tokio/tokio/src/runtime/coop.rs:102:5
    |
102 |     f()
    |     ^^^
note: inside `tokio::runtime::coop::budget::<std::task::Poll<()>, [closure@tokio::runtime::scheduler::current_thread::CoreGuard<'_>::block_on<std::pin::Pin<&mut std::pin::Pin<&mut dyn std::future::Future<Output = ()>>>>::{closure#0}::{closure#0}::{closure#0}]>`
   --> /home/runner/work/tokio/tokio/tokio/src/runtime/coop.rs:68:5
    |
68  |     with_budget(Budget::initial(), f)
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside closure
   --> /home/runner/work/tokio/tokio/tokio/src/runtime/scheduler/current_thread.rs:541:25
    |
541 |                         crate::runtime::coop::budget(|| future.as_mut().poll(&mut cx))
    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `tokio::runtime::scheduler::current_thread::Context::enter::<std::task::Poll<()>, [closure@tokio::runtime::scheduler::current_thread::CoreGuard<'_>::block_on<std::pin::Pin<&mut std::pin::Pin<&mut dyn std::future::Future<Output = ()>>>>::{closure#0}::{closure#0}]>`
   --> /home/runner/work/tokio/tokio/tokio/src/runtime/scheduler/current_thread.rs:350:19
    |
350 |         let ret = f();
    |                   ^^^
note: inside closure
   --> /home/runner/work/tokio/tokio/tokio/src/runtime/scheduler/current_thread.rs:540:36
    |
540 |                       let (c, res) = context.enter(core, || {
    |  ____________________________________^
541 | |                         crate::runtime::coop::budget(|| future.as_mut().poll(&mut cx))
542 | |                     });
    | |______________________^
note: inside closure
   --> /home/runner/work/tokio/tokio/tokio/src/runtime/scheduler/current_thread.rs:615:57
    |
615 |         let (core, ret) = CURRENT.set(&self.context, || f(core, &self.context));
    |                                                         ^^^^^^^^^^^^^^^^^^^^^^
note: inside `tokio::macros::scoped_tls::ScopedKey::<tokio::runtime::scheduler::current_thread::Context>::set::<[closure@tokio::runtime::scheduler::current_thread::CoreGuard<'_>::enter<[closure@tokio::runtime::scheduler::current_thread::CoreGuard<'_>::block_on<std::pin::Pin<&mut std::pin::Pin<&mut dyn std::future::Future<Output = ()>>>>::{closure#0}], std::option::Option<()>>::{closure#0}], (std::boxed::Box<tokio::runtime::scheduler::current_thread::Core>, std::option::Option<()>)>`
   --> /home/runner/work/tokio/tokio/tokio/src/macros/scoped_tls.rs:61:9
    |
61  |         f()
    |         ^^^
note: inside `tokio::runtime::scheduler::current_thread::CoreGuard::<'_>::enter::<[closure@tokio::runtime::scheduler::current_thread::CoreGuard<'_>::block_on<std::pin::Pin<&mut std::pin::Pin<&mut dyn std::future::Future<Output = ()>>>>::{closure#0}], std::option::Option<()>>`
   --> /home/runner/work/tokio/tokio/tokio/src/runtime/scheduler/current_thread.rs:615:27
    |
615 |         let (core, ret) = CURRENT.set(&self.context, || f(core, &self.context));
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `read_buf_bad_async_read`
   --> tokio/tests/io_read.rs:74:5
    |
74  |     BadAsyncRead::new().read_buf(&mut buf).await.unwrap();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside closure
   --> tokio/tests/io_read.rs:72:36
    |
70  | #[tokio::test]
    | -------------- in this procedural macro expansion
71  | #[should_panic]
72  | async fn read_buf_bad_async_read() {
    |                                    ^
    = note: this error originates in the macro `deref_async_read` which comes from the expansion of the attribute macro `tokio::test` (in Nightly builds, run with -Z macro-backtrace for more info)

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to previous error

error: test failed, to rerun pass `--test io_read`

Caused by:
  process didn't exit successfully: `/home/runner/.rustup/toolchains/nightly-2022-12-27-x86_64-unknown-linux-gnu/bin/cargo-miri runner /home/runner/work/tokio/tokio/target/miri/x86_64-unknown-linux-gnu/debug/deps/io_read-9a80d00f6027f2d9` (exit status: 1)
```

tokio/tests/support/leaked_buffers.rs

@@ -18,9 +18,9 @@ impl LeakedBuffers {
         }
     }
     pub unsafe fn create<'a>(&mut self, size: usize) -> &'a mut [u8] {
-        let mut new_mem = vec![0u8; size].into_boxed_slice();
-        let slice = std::slice::from_raw_parts_mut(new_mem.as_mut_ptr(), new_mem.len());
+        let new_mem = vec![0u8; size].into_boxed_slice();
         self.leaked_vecs.push(new_mem);
-        slice
+        let new_mem = self.leaked_vecs.last_mut().unwrap();
+        std::slice::from_raw_parts_mut(new_mem.as_mut_ptr(), new_mem.len())
     }
 }