bug fixed

git-svn-id: http://code.alibabatech.com/svn/druid/trunk@725 b9813039-fb51-4c41-a8b9-e21c2acb5095

src/main/java/com/alibaba/druid/stat/DruidDataSourceStatManager.java

@@ -77,6 +77,10 @@ public class DruidDataSourceStatManager implements DruidDataSourceStatManagerMBe
 
     public synchronized static void remove(DruidDataSource dataSource) {
         ObjectName objectName = dataSources.remove(dataSource);
+        
+        if (objectName == null) {
+            return;
+        }
 
         MBeanServer mbeanServer = ManagementFactory.getPlatformMBeanServer();