vp9: fix bugs in updating coef probabilities with parallelmode=1

- The memcpy was completely wrong because
s->prob_ctx[s->framectxid].coef is a [4][2][2][6][6][3] array, whereas
s->prob.coef is a [4][2][2][6][6][11] array.
- The additional check was committed to ffmpeg by Ronald S. Bultje.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
