ZOOKEEPER-3008: Potential NPE in SaslQuorumAuthLearner#authenticate and SaslQuorumAuthServer#authenticate

src/java/main/org/apache/zookeeper/server/quorum/auth/SaslQuorumAuthLearner.java

@@ -94,7 +94,7 @@ public void authenticate(Socket sock, String hostName) throws IOException {
                     principalConfig,
                     QuorumAuth.QUORUM_SERVER_PROTOCOL_NAME,
                     QuorumAuth.QUORUM_SERVER_SASL_DIGEST, LOG, "QuorumLearner");
-
+            // may happen NPE at here
             if (sc.hasInitialResponse()) {
                 responseToken = createSaslToken(new byte[0], sc, learnerLogin);
             }

src/java/main/org/apache/zookeeper/util/SecurityUtils.java

@@ -135,6 +135,7 @@ public SaslClient run() throws SaslException {
                 return saslClient;
             } catch (Exception e) {
                 LOG.error("Exception while trying to create SASL client", e);
+                // may cause NPE at the caller
                 return null;
             }
         }