[idhyt (idhyt)]

What changed

Use checked_add instead of + when computing the length for
Sequence::deserialize_lazy in deserialize_enum, so that
extremely large DER long-form length values produce a
TruncatedData error instead of wrapping or overflowing.

Why

Length::deserialized can return usize::MAX for a valid-looking
but extremely large DER long-form length field (e.g. 0x88 0xFF…0xFF).
With plain + arithmetic, 1 + usize::MAX wraps (or panics under
overflow checks), resulting in a bogus length being passed downstream.

Before / After

// Before
let len = 1 + payload_len + Length::encoded_len(payload_len);

// After
let len = payload_len
    .checked_add(1)
    .and_then(|v| v.checked_add(Length::encoded_len(payload_len)))
    .ok_or(Asn1DerError::TruncatedData)?;

Validation

• cargo test --lib passed (6 tests, including a new regression test)
• cargo fmt -- --check clean
• cargo clippy --all-targets --all-features -- -D warnings clean
• Added regression test deserialize_enum_rejects_oversized_length
  covering an extremely large DER long-form length field