Security fixes
Three independent bugs have allowed a client to trigger a panic in the session handler task, thereby crashing their own session.
- GHSA-cqjc-rmpq-xprq: sending a malformed PTY request packet - 8912512
- GHSA-g9hv-x236-4qp3: sending a malformed Curve25519 KEX packet - a7fc1eb
- GHSA-5xvq-cp9x-6p6r: sending a zero Curve25519 key - a7fc1eb