Fix invalid index_path crashes

Fix issue where table_cell_data should return nil when an invalid index_path is given
Hmm ... I guess a higher level of what I'm running into is the fact that
I'm trying to access the tableView's cell from the tableView
heightForRowAtIndexPath and its crashing my application.

~~~ ruby

class AllFeed < PM::TableScreen

  def tableView(table_view, heightForRowAtIndexPath:index)
    cell = table_view.cellForRowAtIndexPath index
  end
end

~~~

This crashes my app giving me a memory access error at that location.



* Jamon Holmgren <notifications@github.com> [14-07-10 18:59]:
>> @@ -101,6 +101,7 @@ def delete_row(index_paths, animation = nil)
>>      def table_view_cell(params={})
>>        params = index_path_to_section_index(params)
>>        data_cell = self.promotion_table_data.cell(section: params[:section], index: params[:index])
>> +      return nil if data_cell.nil?
>>        return UITableViewCell.alloc.init unless data_cell
>
>This line will never be reached with the addition of the line above. Should it be returning nil or a blank UITableViewCell like before?
>
>---
>Reply to this email directly or view it on GitHub:
>https://github.com/clearsightstudio/ProMotion/pull/503/files#r14803612