Remote Denial of Service via malformed NIP‑44 v2 payload

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 buffer0..2 without first verifying that the decrypt...