Port "Better workaround for Bug 1089." to vp10 (nextgenv2).
Don't initialize first pass costs for a number of symbols where first pass probabilities aren't initialized. As a side effect, an illegal read in the ANS experiment is fixed. https://bugs.chromium.org/p/webm/issues/detail?id=1089 Change-Id: I97438c357bd88f52f5a15c697031cf0c3cc8f510