vp9: write uveob as 16-bit value for 16x16/32x32 transforms.

This fixes make fate-vp9-00-quantizer-01 THREADS=2.

libavcodec/vp9.c

@@ -2258,8 +2258,12 @@ static void decode_coeffs(AVCodecContext *ctx)
                                   16 * step * step, c, e, p, a[x] + l[y], \
                                   uvscan, uvnb, uv_band_counts, qmul[1]); \
             a[x] = l[y] = !!res; \
+            if (step >= 4) { \
+                AV_WN16A(&s->uveob[pl][n], res); \
+            } else { \
                 s->uveob[pl][n] = res; \
             } \
+        } \
     }
 
     p = s->prob.coef[b->uvtx][1 /* uv */][!b->intra];