Set @report value based on session[:rpt_task_id] or [:report_result_id]

While downloading a PDF for Timelines, @report is nil causing a crash,
which has been fixed.

https://bugzilla.redhat.com/show_bug.cgi?id=1138829
Merge pull request #511 from AparnaKarve/bz1138829_render_pdf_fix_for_timelines

Fixed crash while downloading PDF for Timelines