Disallow 64-byte transactions.

Would make payments exactly 64 bytes long invalid, closing loopholes that can fool light wallets and nodes.

Each block holds a fingerprint of all its payments, built as a tree. A payment of exactly 64 bytes, not counting its SegWit data, can be mistaken for part of that tree, letting attackers trick light wallets or confuse nodes. Banning such payments would close the gap.