When unencrypted secret key packets are serialized, a 2-byte checksum is
appended after the key material. According to rfc4880bis, these 2 bytes are
not included in the length of the key material (this encoded length is a new
addition of rfc4880bis, specific to v5 keys). We erroneously included them,
causing other implementations to fail to parse unencrypted v5 private keys
generated by OpenPGP.js.