dnxhddec: initialize with mb-aligned dimensions

The coded size is a multiple of the macroblock size, which is 16.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dnxhddec.c

@@ -87,6 +87,9 @@ static av_cold int dnxhd_decode_init(AVCodecContext *avctx)
     ctx->cid = -1;
     avctx->colorspace = AVCOL_SPC_BT709;
 
+    avctx->coded_width  = FFALIGN(avctx->width,  16);
+    avctx->coded_height = FFALIGN(avctx->height, 16);
+
     ctx->rows = av_mallocz_array(avctx->thread_count, sizeof(RowContext));
     if (!ctx->rows)
         return AVERROR(ENOMEM);