Fix crash in /submissions/my when not signed in

We need to check that we have a current user before using
current_user.id.

Error trace:

```
NoMethodError (undefined method `id' for nil:NilClass):
  app/controllers/submissions_controller.rb:19:in `index'
```

app/controllers/submissions_controller.rb

@@ -14,6 +14,7 @@ def permitted_params
   # GET /submissions
   # GET /submissions.xml
   def index
+    raise Pundit::NotAuthorizedError if current_user.nil?
     params[:by_user] = current_user.id.to_s if params[:filter] == 'my'
     if !params[:by_user].nil? && params[:by_user].to_i != current_user.id
       authorize User.find(params[:by_user]), :inspect?