extract_from_envelope() doesn't have validation

[artiomn]

extract_from_envelope() in https://github.com/KasperskyLab/knp/blob/master/knp/core-library/impl/messaging/message_envelope.cpp#L63-L78 doesn't have validation.

Possible fix:

flatbuffers::Verifier verifier(buffer, size); 
if (!verifier.VerifyBuffer<marshal::MessageEnvelope>()) {
    throw std::runtime_error("Invalid FlatBuffer data");
}