mjpegdec: tighten unescaped_buf_size size check, prevent null ptr deref

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

libavcodec/mjpegdec.c

@@ -1611,7 +1611,7 @@ 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)) {
+        } else if (unescaped_buf_size > (1U<<28)) {
             av_log(avctx, AV_LOG_ERROR, "MJPEG packet 0x%x too big (0x%x/0x%x), corrupt data?\n",
                    start_code, unescaped_buf_size, buf_size);
             return AVERROR_INVALIDDATA;