GHSA-rcx8-48pc-v9q8: mail-internals use-after-free vulnerability in `vec_insert_bytes`

Incorrect reallocation logic in the function vec_insert_bytes causes a use-after-free.

This function does not have to be called directly to trigger the vulnerability because many methods on EncodingWriter 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.