Use let-else in buf_get_impl

[braddunbar]

I think let-else is a bit more explicit here.

[taiki-e]

Closing because let-else is not available on our MSRV.

Thanks anyway for the PR.

[seanmonstar]

This would require a newer rustc. Even if it were within policy to pick one that supports this syntax, I wouldn't want us to do that just for a syntax change. I lean towards only upgrading MSRV when it will provide a useful improvement to users.

[braddunbar]

Ah! That makes a lot of sense. Thank you for the response! 🙏