vp9: sync threads after a longjmp

Synchronize all threads immediately as a subsequent decode call may
cause a resize invalidating some allocations.

fixes one aspect of crbug.com/437655

https://gerrit.chromium.org/gerrit/73143

BUG=437655

R=tomfinegan@chromium.org

Review URL: https://codereview.chromium.org//848963004
1 file changed