Overflow of frame error accumulators.

This fixes an overflow problem in the frame error accumulators.

The overflow condition is extreme but did trigger when Frank B.
coded some high motion interlaced HD content.

The observed effect was a catastrophic  breakdown of the rate
control leading to massive undershoot and poor bit allocation.

All the error values should really be unsigned but I will look at this
separately.

Change-Id: I9745f5c5ca2783620426b66b568b2088b579151f