Fix #644: BrokerServiceThrottlingTest.testLookupThrottlingForClientByBroker...

Fix #644: BrokerServiceThrottlingTest.testLookupThrottlingForClientByBroker intermittent failures (#645)

pulsar-broker/src/test/java/org/apache/pulsar/broker/service/BrokerServiceThrottlingTest.java

@@ -173,8 +173,10 @@ public class BrokerServiceThrottlingTest extends BrokerTestBase {
         }
         latch.await();
 
-        for (int i = 0; i < successfulConsumers.size(); i++) {
-            successfulConsumers.get(i).close();
+        for (Consumer c : successfulConsumers) {
+            if (c != null) {
+                c.close();
+            }
         }
         pulsarClient.close();
         assertNotEquals(successfulConsumers.size(), totalConsumers);