Protect against Core's Merkle leaf node weakness

Protect against Core's Merkle leaf node weakness

Bitcoin Core's Merkle tree implementation has no way to discern between
internal and leaf node entries. As a consequence it is susceptible to an
attacker injecting additional transactions by crafting 64-byte
transactions matching an inner Merkle node's hash (see
https://web.archive.org/web/20240329003521/https://bitslog.com/2018/06/09/leaf-node-weakness-in-bitcoin-merkle-tree-design/).

To protect against this (highly unlikely attack vector), we check that
the transaction isn't 64 bytes in length, and skip it otherwise.