Attempt to fix cppcheck (ros2#646)

Signed-off-by: Shane Loretz <[email protected]>

rclcpp/src/rclcpp/clock.cpp

@@ -130,10 +130,15 @@ Clock::create_jump_callback(
       rclcpp::Clock::on_time_jump, handler);
   if (RCL_RET_OK != ret) {
     delete handler;
-    handler = NULL;
+    handler = nullptr;
     rclcpp::exceptions::throw_from_rcl_error(ret, "Failed to add time jump callback");
   }
 
+  if (nullptr == handler) {
+    // imposible to reach here; added to make cppcheck happy
+    return nullptr;
+  }
+
   // *INDENT-OFF*
   // create shared_ptr that removes the callback automatically when all copies are destructed
   return rclcpp::JumpHandler::SharedPtr(handler, [this](rclcpp::JumpHandler * handler) noexcept {