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>

libavcodec/vp9.c

@@ -1144,8 +1144,17 @@ static int vp9_decode_frame(AVCodecContext *avctx, AVFrame *frame,
     // probability maintenance between frames)
     if (s->refreshctx) {
         if (s->parallelmode) {
-            memcpy(s->prob_ctx[s->framectxid].coef, s->prob.coef,
-                   sizeof(s->prob.coef));
+            int j, k, l, m;
+            for (i = 0; i < 4; i++) {
+                for (j = 0; j < 2; j++)
+                    for (k = 0; k < 2; k++)
+                        for (l = 0; l < 6; l++)
+                            for (m = 0; m < 6; m++)
+                                memcpy(s->prob_ctx[s->framectxid].coef[i][j][k][l][m],
+                                       s->prob.coef[i][j][k][l][m], 3);
+                if (s->txfmmode == i)
+                    break;
+            }
             s->prob_ctx[s->framectxid].p = s->prob.p;
         } else {
             ff_vp9_adapt_probs(s);