Fix some 'impossible' overflow bugs in byte counting

Fix some 'impossible' overflow bugs in byte counting

The first was genuinely impossible, I think: it could only happen
when the amount we read differed from the amount we wanted to read
by more than INT_MAX.

The second is just very unlikely: it would give incorrect results to
the controller if you somehow wrote or read more than 4GB on one
edge conn in one second.  That one is a bugfix on 0.1.2.8-beta.