avcodec/pngdec: Check nb_blocks

Fixes: Timeout (23sec -> 0.5sec)
Fixes: 14329/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_LSCR_fuzzer-5679252923482112

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/pngdec.c

@@ -1541,6 +1541,8 @@ 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)
+        return AVERROR_INVALIDDATA;
 
     if (s->last_picture.f->data[0]) {
         ret = av_frame_copy(frame, s->last_picture.f);