Do not crash when Kerberos SRV record is not found

ipa-client-install crashed when IPA server realm TXT record was
configured, but the referred domain (lower-case realm value) did
not contain any Kerberos SRV record (_kerberos._udp.<realm>.)

https://fedorahosted.org/freeipa/ticket/3316

ipa-client/ipaclient/ipadiscovery.py

@@ -436,9 +436,10 @@ class IPADiscovery(object):
             kdc = self.ipadns_search_srv(domain, '_kerberos._udp', 88,
                     break_on_first=False)
 
-            if not kdc:
+            if kdc:
+                kdc = ','.join(kdc)
+            else:
                 root_logger.debug("SRV record for KDC not found! Realm: %s, SRV record: %s" % (realm, qname))
                 kdc = None
-            kdc = ','.join(kdc)
 
         return realm, kdc