Fix unused Box linter warning.

[ttsugriy]

cargo check is not happy with original code

    = note: call `drop(Box::from_raw(ptr))` if you intend to drop the `Box`
    = note: `#[warn(unused_must_use)]` on by default
help: use `let _ = ...` to ignore the resulting value
    |
151 |             let _ = Box::from_raw(writing);
    |             +++++++

and adding drop is not only recommended but also makes the intent explicit.