Fix missing variable bug with link dnd code

When using the experimental feature 'drag and drop of links and text', and specifically when dragging a link from one browser window into Moodle, a notice will be seen the the web server logs regarding a missing variable. This isn't seen since the call to create the resource is an async one, however, for the MoodleNet 3.8 plugin to work properly (i.e. NOT generate a notice on screen), we need to patch this.

The fix for this on the master branch is being included as part of the master MoodleNet plugin on MDL-67883, so this is only needed for 3.8.

To replicate:
1. Go to 'Development' -> 'Experimental settings' and enable 'dndallowtextandlinks'
2. Go to a course and turn editing on
3. Open another browser window and go to youtube.com
4. Try to drag a link from the youtube window into the course (just drag a link into the course until you see 'add link here' drop area
5. Continue, setting a name and clicking 'Upload'
6. Review your server's error logs (for apache, just check /var/log/apache2/error.log)
7. See the notice:

PHP Notice:  Undefined property: stdClass::$coursemodule