Fix the size calculation to prefer the shared version if both are available.

endpoints/registry.py

@@ -307,7 +307,7 @@ def get_image_json(namespace, repository, image_id, headers):
 
   profile.debug('Looking up repo layer size')
   try:
-    size = repo_image.image_size or repo_image.storage.image_size
+    size = (repo_image.storage and repo_image.storage.image_size) or repo_image.image_size
     headers['X-Docker-Size'] = str(size)
   except OSError:
     pass