Nimiq: Panic in TrieProof::verify via child_index unwrap on equal-length keys

Nimiq is a Rust implementation of the Nimiq Proof-of-Stake protocol based on the Albatross consensus algorithm. Prior to 1.6.0, a malicious state-sync peer can crash a syncing node by sending a crafted TrieChunk proof containing two TrieProofNode values with identical keys. TrieProof::verify calls TrieProofNode::child_index in primitives/src/trie/trie_proof_node.rs, where is_prefix_of accepts equal keys and KeyNibbles::get is called at the key length, returns None, and is unconditionally unwrapped. Untrusted ResponseChunk data reaches commit_chunks, put_chunk, and proof.verify before cryptographic proof validation, so the attacker does not need a valid proof. Exploitation requires the attacker to be selected as the victim's sync peer during state sync, and the resulting panic is transient because the node restarts and resynchronizes. This issue is fixed in version 1.6.0.