BUG/MINOR: dns: Fix check on nameserver in snr_resolution_cb

snr_resolution_cb can be called with <nameserver> parameter set to NULL. So we
must check it before using it. This is done most of time, except when we deal
with invalid DNS response.
diff --git a/src/server.c b/src/server.c
index ec2dbe8..ebfe0e5 100644
--- a/src/server.c
+++ b/src/server.c
@@ -3990,10 +3990,11 @@
 	return 1;
 
  invalid:
-	if (nameserver)
+	if (nameserver) {
 		nameserver->counters.invalid += 1;
-	if (resolution->nb_responses >= nameserver->resolvers->count_nameservers)
-		goto update_status;
+		if (resolution->nb_responses >= nameserver->resolvers->count_nameservers)
+			goto update_status;
+	}
 
 	snr_update_srv_status(s, has_no_ip);
 	return 0;