OpenPGP - Partial length encoding: Enforce minimum length of the first chunk

[vanitasvitae]

The OpenPGP specification mandates that if partial body encoding is used, the first chunk must be at least 512 bytes long. Currently, BC does not enforce this requirement (see https://tests.sequoia-pgp.org/#Tests_support_for_partial_body_encoding )

This PR adds a check for this requirement, which will throw an IOException for malformed packets.