fix RedirectHandler NullPointerException, when save-result option is true. close #517

core/src/main/java/com/taobao/arthas/core/shell/command/internal/RedirectHandler.java

@@ -50,6 +50,8 @@ public class RedirectHandler extends PlainTextHandler implements CloseFunction {
 
     @Override
     public void close() {
-        out.close();
+        if (out != null) {
+            out.close();
+        }
     }
 }