[rt_media attachment_id=XX] short does not work

Using this shortcode does not show anything and throws PHP Notice for Undefined variable: media_url

Please check this screenshot.

![rtTranscoder_Shortcode_–_krupa-transcoder_dev2_rt_gw](https://user-images.githubusercontent.com/15173772/77638861-d5fdda00-6f7d-11ea-881e-d0089157dc26.png)
