dpx: Fix rounding in 10bit total_size calculation

This could have caused out of array reads
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dpx.c

@@ -159,7 +159,7 @@ static int decode_frame(AVCodecContext *avctx,
                 return -1;
             }
             avctx->pix_fmt = AV_PIX_FMT_GBRP10;
-            total_size = (4 * avctx->width * avctx->height * elements) / 3;
+            total_size = (avctx->width * avctx->height * elements + 2) / 3 * 4;
             break;
         case 12:
             if (!packing) {