StringNarrow uses wcstombs on Microsoft platforms about cryptopp

[noloader]

The Windows code was changed to the following. Its guarded by (CRYPTOPP_MSC_VERSION >= 1400).

    std::string result;
    size_t len=0, size = 0;
    errno_t err = 0;

    const wchar_t* ptr = str;
    while (*ptr++) len++;

    err = wcstombs_s(&size, NULL, 0, str, len*sizeof(wchar_t));
    assert(err == 0);
    if (err != 0) {goto CONVERSION_ERROR;}

    result.resize(size);    
    err = wcstombs_s(&size, &result[0], size, str, size);
    assert(err == 0);

    if (err != 0)
    {
CONVERSION_ERROR:
        if (throwOnError)
            throw InvalidArgument("StringNarrow: wcstombs_s() call failed with error " + IntToString(err));
        else
            return std::string();
    }

    // The safe routine's size includes the NULL.
    if (!result.empty())
        result.resize(size - 1);

    return result;