聚类算法-传入簇参数大于文档个数时报空指针

[Mryang11]

Describe the bug
传入簇参数大于文档个数时报空指针

Code to reproduce the issue
Provide a reproducible test case that is the bare minimum necessary to generate the problem.

public static void main(String[] args) {
		String a = "select * from table;";
		String b = "select * from table;";
		String c = "select * from table where id = 100;";
		String d = "select * from table;";
		String e = "delete from table;";
		String f = "update table set age = 1 where id = 9";
		String g = "update table set age = 1 where id = 88";
		String h = "update table set age = 1 where id = 10";

		Set<Integer> set = new HashSet<>();
		List<String> list = new ArrayList<>();
		list.add(a);
		list.add(b);
		list.add(c);
		list.add(d);
		list.add(e);
		list.add(f);
		list.add(g);
		list.add(h);
		String[] array = list.toArray(new String[0]);
		set.add(0);
		set.add(1);
		set.add(3);
		set.add(5);
		set.add(6);
		set.add(7);
		System.out.println("================聚类算法==============");
		ClusterAnalyzer<String> analyzer = new ClusterAnalyzer<>();
		for (Integer s: set) {
			analyzer.addDocument(String.valueOf(s), array[s]);
		}
		// k大于set集合大小
		int k = 10;
		System.out.println(analyzer.kmeans(k));
		System.out.println();

		System.out.println(analyzer.repeatedBisection(k));
		System.out.println(analyzer.repeatedBisection(1.0));

	}

Describe the current behavior

kmeans:
================聚类算法==============
Exception in thread "main" java.lang.NullPointerException
	at com.hankcs.hanlp.mining.cluster.ClusterAnalyzer.refine_clusters(ClusterAnalyzer.java:263)
	at com.hankcs.hanlp.mining.cluster.ClusterAnalyzer.kmeans(ClusterAnalyzer.java:147)

repeatedBisection:
================聚类算法==============

Exception in thread "main" java.lang.NullPointerException
	at com.hankcs.hanlp.mining.cluster.ClusterAnalyzer.repeatedBisection(ClusterAnalyzer.java:222)
	at com.hankcs.hanlp.mining.cluster.ClusterAnalyzer.repeatedBisection(ClusterAnalyzer.java:180)

Expected behavior
簇类参数传的值越界时，程序应当友好处理为文档个数
System information

• OS Platform and Distribution (e.g., Linux Ubuntu 16.04): Mac OS
• Python version:
• HanLP version: 1.7.6

Other info / logs
Include any logs or source code that would be helpful to diagnose the problem. If including tracebacks, please include the full traceback. Large logs and files should be attached.

• I've completed this form and searched the web for solutions.

[hankcs]

感谢反馈，已经修复，请参考上面的commit。
如果还有问题，欢迎重开issue。