Prevent panic in MerkleTree MultiPath::verify by handling missing leaves

[anim001k]

Description (Markdown):

• What: Replace unwrap() in MultiPath::verify with explicit check and return Err(Error::IncorrectInputLength(...)) when the iterator of leaves is shorter than leaf_indexes.len().
• Why: Avoids potential panic/DoS on malformed or adversarial input; aligns with the function’s Result<bool, Error> contract; consistent with Path::verify behavior.
• Build: cargo check passes.