fix error because course not have end time

app/views/users/_course_exercise.html.erb

@@ -28,7 +28,7 @@
                 <%= link_to '立即发布', publish_notice_exercise_path(activity), :remote => true %>
               </li>
             <% end %>
-            <% if (activity.exercise_status == 2 && activity.end_time > Time.now) || activity.exercise_group_settings.where("publish_time < '#{Time.now}' and end_time > '#{Time.now}'").count > 0 %>
+            <% if (activity.exercise_status == 2 && (activity.end_time.blank? || activity.end_time > Time.now)) || activity.exercise_group_settings.where("publish_time < '#{Time.now}' and end_time > '#{Time.now}'").count > 0 %>
               <li>
                 <%= link_to '立即截止', end_notice_exercise_path(activity), :remote => true %>
               </li>