Fix cpplint on xenial (#556)

* Change variable name to fix cpplint on xenial

* Set variable to null to satisfy cpplint

* additional null

rclcpp/src/rclcpp/clock.cpp

@@ -130,6 +130,7 @@ Clock::create_jump_callback(
       rclcpp::Clock::on_time_jump, handler);
   if (RCL_RET_OK != ret) {
     delete handler;
+    handler = NULL;
     rclcpp::exceptions::throw_from_rcl_error(ret, "Failed to add time jump callback");
   }
 
@@ -139,6 +140,7 @@ Clock::create_jump_callback(
     rcl_ret_t ret = rcl_clock_remove_jump_callback(&rcl_clock_, rclcpp::Clock::on_time_jump,
         handler);
     delete handler;
+    handler = NULL;
     if (RCL_RET_OK != ret) {
       RCUTILS_LOG_ERROR("Failed to remove time jump callback");
     }