Fix "the client has already been closed" bug (#75)

Fix the problem "Caused by: java.lang.IllegalStateException:
Cannot proceed, the client has already been closed" when 
running in Flink local mode
diff --git a/flink-connector-kudu/src/main/java/org/apache/flink/connectors/kudu/batch/KuduInputFormat.java b/flink-connector-kudu/src/main/java/org/apache/flink/connectors/kudu/batch/KuduInputFormat.java
index 3a35e6a..98877d8 100644
--- a/flink-connector-kudu/src/main/java/org/apache/flink/connectors/kudu/batch/KuduInputFormat.java
+++ b/flink-connector-kudu/src/main/java/org/apache/flink/connectors/kudu/batch/KuduInputFormat.java
@@ -98,6 +98,7 @@
         }
         if (kuduReader != null) {
             kuduReader.close();
+            kuduReader = null;
         }
     }