gracefully handle missing keyspace argument (cli)

Patch by Hafsteinn Baldvinsson; reviewed by eevans for CASSANDRA-551

git-svn-id: https://svn.apache.org/repos/asf/incubator/cassandra/trunk@835990 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/cassandra/cli/CliClient.java

@@ -251,8 +251,14 @@ public class CliClient
         // Get table name
         int childCount = ast.getChildCount();
         assert(childCount == 1);
+
         String tableName = ast.getChild(0).getText();
-        
+
+        if( tableName == null ) {
+            css_.out.println("Keyspace argument required");
+            return;
+        }
+
         // Describe and display
         Map<String, Map<String, String>> columnFamiliesMap;
         try {