fix the number_index is null

Pls, PR it.

```
[Illuminate\Database\QueryException]
  SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'number_index' cannot be null (SQL: upda
  te `discussions` set `posts_count` = 38, `start_post_id` = 1404, `last_time` = 2014-05-14 19:49:12, `
  last_user_id` = 12, `last_post_id` = 1442, `last_post_number` = , `number_index` =  where `id` = 72)


  [PDOException]
  SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'number_index' cannot be null
```