Fix CI failure

[taiki-e]

This fixes CI failure due to new rust lint.

warning: unnecessary allocation, use `&mut` instead
   --> tests/test_buf_mut.rs:247:5
    |
247 |     Box::new(Special).put_u8(b'x');
    |     ^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(unused_allocation)]` on by default

warning: unnecessary allocation, use `&mut` instead
   --> tests/test_buf.rs:102:16
    |
102 |     assert_eq!(Box::new(Special).get_u8(), b'x');
    |                ^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(unused_allocation)]` on by default