Special characters in caption lead to failure of inserting images

[TobiasBg]

Found this when double-checking #22132:

1. Go to add a new post.
2. Click "Upload/Insert" (the "old" media upload).
3. Upload an image, or go to choose one from the media library.
4. Insert Title"<script>alert('Title');</script> in the "Caption" field.
5. Click "Insert into Post".

Instead of the image (or Shortcode) being added to the editor (with a somehow escaped caption field), the media upload iframe just gets a new content:

[/caption]'); /* ]]> */

Reproduced in 3.4.2 and trunk.

screenshot of the error

I was able to reproduce the error. It also allowed the alert on the front end. I am not sure if this will need to be addressed once the new media update is fully implemented.