static int64_t update_size(AVIOContext *pb, int64_t pos)
{
    int64_t curpos = avio_tell(pb);
    avio_seek(pb, pos, SEEK_SET);
    avio_wb32(pb, curpos - pos); /* rewrite size */
    avio_seek(pb, curpos, SEEK_SET);

    return curpos - pos;
}

The product divides a value by zero.