FIX(server): Validate the correct certifiacte from a clients chain

While validating a clients certifiacte, the last certifiacte in the clients chain was used. The last certifiacte is the certifiacte of the CA. Since in this place, the clients own certifiacte should be validated, the first certifiacte in the chain needs to be used instead.

Fixes mumble-voip#3523 (partially)

src/murmur/Server.cpp

@@ -1511,7 +1511,7 @@ void Server::encrypted() {
 
 	QList< QSslCertificate > certs = uSource->peerCertificateChain();
 	if (!certs.isEmpty()) {
-		const QSslCertificate &cert = certs.last();
+		const QSslCertificate &cert = certs.first();
 		uSource->qslEmail           = cert.subjectAlternativeNames().values(QSsl::EmailEntry);
 		uSource->qsHash             = QString::fromLatin1(cert.digest(QCryptographicHash::Sha1).toHex());
 		if (!uSource->qslEmail.isEmpty() && uSource->bVerified) {