Fixes issue where Cells are Missing after updateVisibleCellsNow

In some cases where the cells are sufficiently large and `showsHorizontalScrollIndicator` and `showsVerticalScrollIndicator` are both `NO` the number of subviews becomes zero. If this happens and `self.dragging` is `YES` then we are trying to insert the cell at a negative index which will not add the subview. Any subsequent insertion will also fail since the number of subviews will still be zero. This fix ensures that we insert the cell at a non-negative index.