fix bug when parsing spreadsheets with text box

@tealeg This is obviously an edge case but I found that when parsing an XLSX file that contains a text box, the library panics. I think this spreadsheet would work as an example:

https://docs.google.com/spreadsheets/d/1T5ZgUts1JFUBQnD8VnomfTV4yVfeNOBEfOQlRlW2zQQ/edit?usp=sharing

This patch fixes it for me. I have to admit I am not entirely sure why, so I hope the other tests pass still.