DoH: Fixing panic in case if there's no response (#2577)

* Fixing panic in case if there's no response

There could be a situation when there's no response after ServeDNS call. With the current implementation, this leads to panic.

* Add comment

core/dnsserver/server_https.go

@@ -119,6 +119,14 @@ func (s *ServerHTTPS) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	// We should expect a packet to be returned that we can send to the client.
 	s.ServeDNS(context.Background(), dw, msg)
 
+	// See section 4.2.1 of RFC 8484.
+	// We are using code 500 to indicate an unexpected situation when the chain
+	// handler has not provided any response message.
+	if dw.Msg == nil {
+		http.Error(w, "No response", http.StatusInternalServerError)
+		return
+	}
+
 	buf, _ := dw.Msg.Pack()
 
 	mt, _ := response.Typify(dw.Msg, time.Now().UTC())