avcodec/pngdec: consider chunk size in minimal size check

assuming each block contains an empty chunk there has to be at least 8 bytes extra.

Fixes: 15327/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_LSCR_fuzzer-5676669303521280
Fixes: Timeout (11->5sec)

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/pngdec.c

@@ -1547,7 +1547,7 @@ static int decode_frame_lscr(AVCodecContext *avctx,
         return ret;
 
     nb_blocks = bytestream2_get_le16(gb);
-    if (bytestream2_get_bytes_left(gb) < 2 + nb_blocks * 12)
+    if (bytestream2_get_bytes_left(gb) < 2 + nb_blocks * (12 + 8))
         return AVERROR_INVALIDDATA;
 
     if (s->last_picture.f->data[0]) {