Allow more reliable detection of truncated bitstreams by being more
precise with the count of "virtual" bits in the value buffer.
Specifically, the VP8_LOTS_OF_BITS value is accumulated into count,
rather than being assigned, which was losing the prior value,
increasing the required tolerance when testing for the error condition.

Change-Id: Ib5172eaa57323b939c439fff8a8ab5fa38da9b69