cdgraphics: Fix out of array write

Fixes Ticket1359
Found-by: Piotr Bandurski <ami_stuff@o2.pl>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/cdgraphics.c

@@ -280,6 +280,10 @@ static int cdg_decode_frame(AVCodecContext *avctx,
         av_log(avctx, AV_LOG_ERROR, "buffer too small for decoder\n");
         return AVERROR(EINVAL);
     }
+    if (buf_size > CDG_HEADER_SIZE + CDG_DATA_SIZE) {
+        av_log(avctx, AV_LOG_ERROR, "buffer too big for decoder\n");
+        return AVERROR(EINVAL);
+    }
 
     ret = avctx->reget_buffer(avctx, &cc->frame);
     if (ret) {