mail-internals use-after-free vulnerability in `vec_insert_bytes`

Summary

Incorrect reallocation logic in the function [`vec_insert_bytes`](https://docs.rs/mail-internals/0.2.3/mail_internals/utils/fn.vec_insert_bytes.html) causes a use-after-free. This function does not have to be called directly to trigger the vulnerability because many methods on [`EncodingWriter`](https://docs.rs/mail-internals/0.2.3/mail_internals/encoder/struct.EncodingWriter.html) call this function internally. The mail-\* suite is unmaintained and the upstream sources have been actively vandalised. A fixed `mail-internals-ng` (and `mail-headers-ng` and `mail-core-ng`) crate has been published which fixes this, and a dependency on another unsound crate.