PANIC on serialize_openssh()

[lambdalisue]

Thanks for the library. It saved me. However, I found that serialize_openssh() panic with the following error

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', /Users/alisue/.asdf/installs/rust/1.54.0/registry/src/git.luolix.top-1ecc6299db9ec823/oss
hkeys-0.5.1/src/format/ossh_privkey.rs:321:42
stack backtrace:
   0: rust_begin_unwind
             at /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/std/src/panicking.rs:515:5
   1: core::panicking::panic_fmt
             at /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/core/src/panicking.rs:92:14
   2: core::panicking::panic
             at /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/core/src/panicking.rs:50:5
   3: core::option::Option<T>::unwrap
             at /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/core/src/option.rs:388:21
   4: osshkeys::format::ossh_privkey::encode_key
             at /Users/alisue/.asdf/installs/rust/1.54.0/registry/src/git.luolix.top-1ecc6299db9ec823/osshkeys-0.5.1/src/format/ossh_privkey.rs:321:29
   5: osshkeys::format::ossh_privkey::encode_ossh_priv
             at /Users/alisue/.asdf/installs/rust/1.54.0/registry/src/git.luolix.top-1ecc6299db9ec823/osshkeys-0.5.1/src/format/ossh_privkey.rs:259:5
   6: osshkeys::format::ossh_privkey::serialize_ossh_privkey
             at /Users/alisue/.asdf/installs/rust/1.54.0/registry/src/git.luolix.top-1ecc6299db9ec823/osshkeys-0.5.1/src/format/ossh_privkey.rs:190:15
   7: osshkeys::keys::KeyPair::serialize_openssh
             at /Users/alisue/.asdf/installs/rust/1.54.0/registry/src/git.luolix.top-1ecc6299db9ec823/osshkeys-0.5.1/src/keys/mod.rs:342:16
   8: rust_osshkeys_panic::main
             at ./src/main.rs:8:9
   9: core::ops::function::FnOnce::call_once
             at /rustc/a178d0322ce20e33eac124758e837cbd80a6f633/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

With the following code

use anyhow::Result;

fn main() -> Result<()> {
    let private_key_str = include_str!("id_rsa");
    let keypair = osshkeys::keys::KeyPair::from_keystr(&private_key_str, None)?;
    println!(
        "{}",
        keypair.serialize_openssh(None, osshkeys::cipher::Cipher::Null)?
    );
    Ok(())
}

With the following id_rsa

-----BEGIN OPENSSH PRIVATE KEY-----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=
-----END OPENSSH PRIVATE KEY-----