avcodec/libxvid: fix potential integer overflow

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/libxvid.c

@@ -713,7 +713,7 @@ static int xvid_encode_frame(AVCodecContext *avctx, AVPacket *pkt,
     xvid_enc_frame_t xvid_enc_frame = { 0 };
     xvid_enc_stats_t xvid_enc_stats = { 0 };
 
-    if ((ret = ff_alloc_packet2(avctx, pkt, mb_width*mb_height*MAX_MB_BYTES + FF_MIN_BUFFER_SIZE)) < 0)
+    if ((ret = ff_alloc_packet2(avctx, pkt, mb_width*(int64_t)mb_height*MAX_MB_BYTES + FF_MIN_BUFFER_SIZE)) < 0)
         return ret;
 
     /* Start setting up the frame */