The library will panic with messages containing corrupted eml attachments

[cosarara]

This test when added to tests/integration_tests.rs showcases the problem:

#[test]
fn corrupted_attachments_should_not_panic() {
    let input = br#"From: Test <test@example.org>
Date: Mon, 1 Oct 2025 01:02:03 +0200
Subject: test
To: test@example.org
Content-Type: multipart/mixed; boundary="000000000000e07f5706407d1689"

--000000000000e07f5706407d1689
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

bla bla bla

--000000000000e07f5706407d1689
Content-Type: message/rfc822
Content-Disposition: attachment; filename="corrupted.eml"
Content-Transfer-Encoding: 8bit

this attachment is not a valid eml, sorry!
--000000000000e07f5706407d1689--
"#;
    let message = MessageParser::default()
        .with_minimal_headers()
        .parse(&input)
        .unwrap();

    let attachments: Vec<_> = message.attachments().collect();

    let sub_message = match &attachments[0].body {
        mail_parser::PartType::Message(m) => m,
        _ => panic!(),
    };

    // this panics with `index out of bounds: the len is 0 but the index is 0`
    // other methods on Message panic the same way
    if let Some(s) = sub_message.attachment_name() {
        println!("name: {}", s);
    }
    // this assertion fails, and that is why it panics
    // (the code everywhere assumes parts[0] will exist)
    assert_ne!(sub_message.parts.len(), 0);
}

running 1 test
test corrupted_attachments_should_not_panic ... FAILED

failures:

---- corrupted_attachments_should_not_panic stdout ----

thread 'corrupted_attachments_should_not_panic' panicked at src/core/header.rs:606:19:
index out of bounds: the len is 0 but the index is 0