Remote Denial of Service via malformed NIP‑44 v2 payload

Summary

The NIP-44 v2 decryption path in the `nostr` crate contains a reachable panic when processing a short or empty ciphertext. After the HMAC check passes and the ciphertext is decrypted via ChaCha20, the code reads a 2‑byte unpadded‑length prefix via `buffer[0..2]` without first verifying that the decrypted buffer contains at least 2 bytes. A malicious sender who holds the symmetric conversation key (e.g., a direct‑message sender) can craft a payload that produces a 0 or 1‑byte decrypted buffer, causing an index‑out‑of‑bounds panic. This can be triggered remotely through any relay that delivers the crafted event to the victim's client, resulting in a denial of service. No key material, plaintext, or memory corruption occurs.

The vulnerability is present in all versions from `0.26.0` up to `0.44.4` (inclusive) and in the alpha releases `0.45.0‑alpha.1` through `0.45.0‑alpha.4`. Versions `0.44.5` and `0.45.0‑alpha.5` contain the fix.

## Credit

Discovered and responsibly disclosed by **Muhammed Shekho** ([mhd-shekho.com](https://mhd-shekho.com)).