Fix item place error when load more

When I use this StaggeredGridView, I add a footer view to show when loading more data from server. When load complete, the items placed like this: 
![screenshot_2014-12-23-14-34-23](https://cloud.githubusercontent.com/assets/10275907/5535191/095edfae-8ab2-11e4-8846-b4106407aafa.jpeg)
the last item should place at right side, but it came to left side. So I check the code, find out the problem is like this: before load more, I have 20 items and a footer in this view, that's total 21 items. The footer's position is 20. So when I load more from server, there is only one item loaded from server and added to the view, and it's position is 20, the GridItemRecord at position 20 exist, it's a footer, and it's column is 0 due to uninitialized.
So the item is placed at left side(column 0).