Fix message size check for max block sizes much smaller than max protocol message size

Summary:
This fixes a subtle bug: if max block size * 2 is less than max protocol message size, messages could be rejected.
This should not be an issue unless or until MAX_PROTOCOL_MESSAGE_LENGTH is made much larger.

Test Plan:

ninja check check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16343