Potential DoS based on unused preprocesses

[kayabaNerve]

Parties who submit a late preprocess do not have their preprocess used. Accordingly, their contents go unchecked, enabling them to place the maximum amount of bytes there (as spam) without detection.

This is also possible upon the re-attempt for sessions which were completed and accordingly won't be re-attempted.

[kayabaNerve]

Would be resolvable by #588.