Fixed crash while downloading PDF for Timelines

While downloading a PDF for Timelines, @report was nil causing a crash, which has been addressed.

https://bugzilla.redhat.com/show_bug.cgi?id=1138829

P.S. The code added in ```render_pdf``` has been kept consistent with the other "sibling" methods - ```render_txt``` and ```render_csv```.



Don't think you need to check for `session[:rpt_task_id].nil?` in the elsif since you are checking for `session[:rpt_task_id]` in the if.