Make X509 serial number parsing code compatible with YARA (#954)

[xbabka01]

Issue: #954

[metthal]

This is just a nitpick but would it be possible to rather do early return from the function, than having several tens of lines of codes indented like this? What I mean by that is that instead of

ASN1_INTEGER* serial_number_asn1 = X509_get_serialNumber(const_cast<X509*>(cert));
if (serial_number_asn1) {
    // ...
}

return {};

you'd have

ASN1_INTEGER* serial_number_asn1 = X509_get_serialNumber(const_cast<X509*>(cert));
if (!serial_number_asn1)
    return {};

// ...

[xbabka01]

Changed as requested

[metthal]

I've merged the PR but I guess we'll also need changes to https://github.com/avast/retdec-regression-tests/ because some test will fail because of this change. Once TC runs it, you'll exactly know which one.