Fix: invalid memory address or nil pointer dereference on empty/broken/lost DNS answer over UDP.

VERSION

-0.5.8
+0.5.9

dns_utils.go

@@ -57,8 +57,8 @@ func NameserverHasChallenge(nameserver, fqdn, challenge string) bool {
 	// request all TXT records
 	TXTAnswer, _, err := client.Exchange(&msg, nameserver)
 
-	// if message is too big, retry using TCP
-	if TXTAnswer.MsgHdr.Truncated {
+	// if message is empty/broken/lost or too big, retry over TCP
+	if TXTAnswer == nil || TXTAnswer.MsgHdr.Truncated {
 		// Set DNS query protocol to TCP to allow for large anwsers
 		client.Net = "tcp"
 		// ask DNS about challenge record