text: Title: HTTP 500 on get permissions (ValueError)
  Body: **Steps to reproduce**
  _docker run -p 8888:8888 kinto/kinto-server_
  Running kinto 14.0.1.dev0.
  
  Request
  ```
  GET /v1/permissions?_since=6148&_token= HTTP/1.1
  Host: 127.0.0.1:8888
  ```
  Response
  ```
  {
      "code": 500,
      "errno": 999,
      "error": "Internal Server Error",
      "message": "A programmatic error occured, developers have been informed.",
      "info": "https://github.com/Kinto/kinto/issues/"
  }
  ```
  
  Log:
  ```
  "GET /v1/permissions?_since=6148&_token=" ? (? ms) not enough values to unpack (expected 3, got 2) errno=999
  
  File "/app/kinto/core/events.py", line 157, in tween
  File "/usr/local/lib/python3.7/site-packages/pyramid/router.py", line 148, in handle_request
  registry, request, context, context_iface, view_name
  File "/usr/local/lib/python3.7/site-packages/pyramid/view.py", line 683, in _call_view
  response = view_callable(context, request)
  File "/usr/local/lib/python3.7/site-packages/pyramid/config/views.py", line 169, in __call__
  return view(context, request)
  File "/usr/local/lib/python3.7/site-packages/pyramid/config/views.py", line 188, in attr_view
  File "/usr/local/lib/python3.7/site-packages/pyramid/config/views.py", line 214, in predicate_wrapper
  File "/usr/local/lib/python3.7/site-packages/pyramid/viewderivers.py", line 325, in secured_view
  File "/usr/local/lib/python3.7/site-packages/pyramid/viewderivers.py", line 436, in rendered_view
  result = view(context, request)
  File "/usr/local/lib/python3.7/site-packages/pyramid/viewderivers.py", line 144, in _requestonly_view
  response = view(request)
  File "/usr/local/lib/python3.7/site-packages/cornice/service.py", line 590, in wrapper
  response = view_()
  File "/app/kinto/core/resource/__init__.py", line 350, in plural_get
  return self._plural_get(False)
  File "/app/kinto/core/resource/__init__.py", line 393, in _plural_get
  include_deleted=include_deleted,
  File "/app/kinto/views/permissions.py", line 77, in get_objects
  parent_id=parent_id,
  File "/app/kinto/views/permissions.py", line 109, in _get_objects
  perms_by_object_uri = backend.get_accessible_objects(principals)
  File "/app/kinto/core/decorators.py", line 45, in decorated
  result = method(self, *args, **kwargs)
  File "/app/kinto/core/permission/memory.py", line 101, in get_accessible_objects
  _, object_id, permission = key.split(":", 2)
  ValueError: not enough values to unpack (expected 3, got 2)
  
  "GET /v1/permissions?_since=6148&_token=" 500 (4 ms) agent=python-requests/2.24.0 authn_type=account errno=999 time=2020-12-21T11:49:07.494000 uid=admin
  ```