fopen return NULL

while (m_continue.load(std::memory_order_relaxed))
{
    // ....
    std::ifstream input(*current_filename, std::ios_base::in);
    if (!input.is_open())
    {
        continue;
    }
    std::ofstream output(*current_filename + ".gz", std::ios_base::out | std::ios_base::binary);
    if (!output.is_open())
    {
        continue;
    }
    boost::iostreams::filtering_streambuf<boost::iostreams::input> in;
    in.push(boost::iostreams::gzip_compressor(m_compress_level));
    in.push(boost::iostreams::counter());
    in.push(input);
    boost::iostreams::copy(in, output);
    output.close();
 
    detail::FileMd5Stat md5_file;
    std::string md5_hash;
    md5_file.get_file_md5(*current_filename + ".gz", md5_hash);
 
    // ....
}

void get_file_md5(const std::string &file_path, std::string &md5_str_result)
{
    unsigned char md5[MD5_DIGEST_LENGTH];
 
    auto file = fopen(file_path.c_str(), "rb");
    assert(file != nullptr && "fopen return null");  <- срабатывает ассерт
    size_t read = 0;
    std::vector<char> buffer(m_block_size);
    while ((read = fread(buffer.data(), sizeof(char), buffer.size() / sizeof(char), file)) != 0)
    {
        MD5_Update(&m_cmd4_ctx, buffer.data(), read);
    }
    MD5_Final(md5, &m_cmd4_ctx);
    md5_str_result.clear();
    boost::algorithm::hex_lower(md5, md5 + MD5_DIGEST_LENGTH, std::back_inserter(md5_str_result));
}