avcodec/vp8: Do not use num_coeff_partitions in thread/buffer setup

The variable is not a constant and can lead to race conditions

Fixes: repro.webm (not reproducable with FFmpeg alone)

Found-by: Dale Curtis <dalecurtis@google.com>
Tested-by: Dale Curtis <dalecurtis@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
