sack_filter: sack_filter_reject underflow guard

[thebugfixers_pm.me]

Summary

If two board entries share the same .end or .start the loop clears both bits and decrements sf_used twice.
Protect by using MPASS(sf->sf_used > 0); in front of the decrement.

[tuexen]

I think this patch would just hide bugs related to the accounting done using sf_used. There are four occurrences of sf->sf_used--. You might want to protect all of them by adding MPASS(sf->sf_used > 0); in front of the decrement.

[thebugfixers_pm.me]

Thanks for the review. Fair point. Revised diff and summary accordingly.