dnxhddec: fix integer overflow / index check

Fixes out of array read

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dnxhddec.c

@@ -180,7 +180,7 @@ static int dnxhd_decode_header(DNXHDContext *ctx, const uint8_t *buf, int buf_si
     for (i = 0; i < ctx->mb_height; i++) {
         ctx->mb_scan_index[i] = AV_RB32(buf + 0x170 + (i<<2));
         av_dlog(ctx->avctx, "mb scan index %d\n", ctx->mb_scan_index[i]);
-        if (buf_size < ctx->mb_scan_index[i] + 0x280) {
+        if (buf_size < ctx->mb_scan_index[i] + 0x280LL) {
             av_log(ctx->avctx, AV_LOG_ERROR, "invalid mb scan index\n");
             return -1;
         }