fix typo in TxOut#to_null_payload
fix serialization roundtrip for invalid script opcodes

   found in testnet3 transaction: 75171dbfa934ff5d163cdbdfb439db067286d2b35114c90edffd4fe9b85aabb7
adjust OP_OP_CHECKMULTISIG

  found in testnet3 transaction: 76755b27e14bf737a418ab2891ebf0414b8643996aa23c38873b66d9ae9e9b9f (last input)