Username length information can be wrong

The length information of the user name can be wrong so that it's value can be `null` after leaving the constructor which then leads to `NullReferenceException`s all over the place (in my case I encountered one while accessing the `DataSize` getter. 

I'm pretty sure this situation is due to a malformed `HSSFWorkbook` file. I'm currently writing a class for conversion between `XSSF`<->`HSSF` files. While the `XSSF->HSSF` part seems to work (at least sufficiently for my use case) reading the converted `HSSFWorkbook` back into an `XSSFWorkbook` got me this `NullReferenceException`. 

I guess it's not really the fault of `HSSF.Record.FileSharingRecord` but rather a problem with the `XSSF->HSSF` part but I think catching such cases isn't a bad idea anyways?
Merge pull request #14 from vanhelgen/patch-1

Username length information can be wrong