Fix stacked borrows violation in get2_unchecked_mut

[taiki-e]

error: Undefined Behavior: trying to reborrow <2754> for Unique permission at alloc1294[0x0], but that tag does not exist in the borrow stack for this location
   --> /Users/taiki/projects/sources/tokio-rs/slab/src/lib.rs:835:16
    |
835 |         match (&mut *ptr1, &mut *ptr2) {
    |                ^^^^^^^^^^
    |                |
    |                trying to reborrow <2754> for Unique permission at alloc1294[0x0], but that tag does not exist in the borrow stack for this location
    |                this error occurs as part of a reborrow at alloc1294[0x0..0x10]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the 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
            
    = note: inside `slab::Slab::<i32>::get2_unchecked_mut` at /Users/taiki/projects/sources/tokio-rs/slab/src/lib.rs:835:16
note: inside `main::_doctest_main_src_lib_rs_820_0` at src/lib.rs:10:33
   --> src/lib.rs:827:33
    |
10  | let (value1, value2) = unsafe { slab.get2_unchecked_mut(key1, key2) };
    |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `main` at src/lib.rs:14:3
   --> src/lib.rs:831:3
    |
14  | } _doctest_main_src_lib_rs_820_0() }
    |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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

error: aborting due to previous error

[taiki-e]

@Darksonn Thanks for the review! I applied your suggestions.