如果zk上canal server cluster子节点为空，日志无法打出，有内存溢出风险

[lanicc]

canal/client/src/main/java/com/alibaba/otter/canal/client/impl/ClusterCanalConnector.java

Line 68 in 80ca436

logger.warn("failed to connect to:{} after retry {} times", accessStrategy.currentNode(), times);

accessStrategy.currentNode()会报错，导致currentConnector.disconnect()未调用，发生内存泄漏，逻辑如下

[lanicc]

connect没成功的情况下，如果不调用disconnect就会内存溢出