Update orm.go

This change is required to prevent crash for the following struct when trying to `LoadRelated` **ToUser**:


```
type FriendRequest struct {
	Id       int64     `orm:"pk;auto"`
	FromUser *AuthUser `orm:"rel(fk)"`
	ToUser   *AuthUser `orm:"rel(fk)"`
	Accepted bool
	Rejected bool

	Created time.Time `orm:"auto_now_add"`
	Updated time.Time `orm:"auto_now"`
}
```

Even by using rel(one) for both also crashes.