[Node.js] Fix the messy code of notification description
[Node.js] Fix the messy code of notification description

Description string is duplicated when used out of the
notification callback as it would be freed by librealsense.

IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense