Fixes tsan warnings like this in fate-vp8-test-vector-007:

WARNING: ThreadSanitizer: data race (pid=65909)
  Write of size 4 at 0x7d8c0000e088 by thread T1:
    #0 vp8_decode_mb_row_sliced vp8.c:2519 (ffmpeg:x86_64+0x100995ede)
[..]
  Previous write of size 4 at 0x7d8c0000e088 by thread T2:
    #0 vp8_decode_mb_row_sliced vp8.c:2519 (ffmpeg:x86_64+0x100995ede)