Fix a panic if Noise frame length is too low

[tomaka]

The value in expected_len is read from an incoming networking packet. The networking packet is invalid if expected_len < 16, however it is unwise to do expected_len - 16 as a malicious peer might send an invalid value on purpose.

Rust only detects underflow errors in debug mode. However, in release mode this will lead to a very high value being passed to resize, which will lead to an allocation error.

This bug is technically a vulnerability, as anyone can crash a smoldot light client if it connects to their node. However, since we're not widely used at the moment and that an attacker has nothing to gain except being a nuisance, I'm going to assume that nobody is going to exploit this and skip the non-disclosure period.

[mergify]

Automatically approving tomaka's pull requests. This auto-approval will be removed once more maintainers are active.

[tomaka]

Note that this changes the order of operations. Before this PR, we add len_before and expected_len, then subtract 16. After this PR, we first subtract 16. This is intentional and not very important.

[github-actions]

twiggy diff report

Difference in .wasm size before and after this pull request.

---

 Delta Bytes │ Item
─────────────┼──────────────────────────────────────────────────────────────────────────────────
         +10 ┊ smoldot::libp2p::connection::noise::Noise::inject_inbound_data::hbeba5cb020cfd9ec
         +10 ┊ Σ [1 Total Rows]

[melekes]

👍