mjpeg: abort decoding if packet is too large.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/mjpegdec.c

@@ -1458,6 +1458,10 @@ int ff_mjpeg_decode_frame(AVCodecContext *avctx, void *data, int *data_size,
         /* EOF */
         if (start_code < 0) {
             goto the_end;
+        } else if (unescaped_buf_size > (1U<<29)) {
+            av_log(avctx, AV_LOG_ERROR, "MJPEG packet 0x%x too big (0x%x/0x%x), corrupt data?\n",
+                   start_code, unescaped_buf_ptr, buf_size);
+            return AVERROR_INVALIDDATA;
         } else {
             av_log(avctx, AV_LOG_DEBUG, "marker=%x avail_size_in_buf=%td\n",
                    start_code, buf_end - buf_ptr);