Fix bug that caused really-long sub-board load times.
I'm not sure how long this has been like this, but the last_msg of a sub-board can be a super high number (e.g. in the billions), so using that value as the highest message offset would cause tons of message index read failures and just cause this loop to interate unneccessarily through tons of non-existent messages. Also, this code is utter crap.
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