Saving an existing premium product breaks the image URLs

Steps to reproduce:

1. Begin with CiviCRM 4.7.22 (master) on Drupal 7
2. Go to /civicrm/admin/contribute/managePremiums?reset=1
3. Add Premium
   1. Name: "Foo"
   2. Premium Image: "Upload from my computer" > (browse to upload an image file)
   3. Minimum Contribution Amount: 5
   4. Market Value: 5
   5. Save
4. Foo > Edit
5. Expect and observe that the values for "Image URL" and "Thumbnail URL" both begin with /sites/default/files/civicrm
6. Make no changes to the form
7. Save
8. Foo > Edit (again)
9. Expect the same values for "Image URL" and "Thumbnail URL"
10. Observe that these values now begin with /files/civicrm instead of /sites/default/files/civicrm

Implications:

• Any edits to premium products will break the display of images

Workaround:

1. When editing a premium product, prepend both URL fields with /a/a