Fixed a DataSource regression when calculating Content-Range.dtcp.com

preroll_static_resource() was using the first content offset in the
 range_offset_list (range_offset_list[0]) instead of the last - causing
 a couple LPTT tests to fail. This regression was probably introduced
 a while back but not caught until now.

Ran the same Range.dtcp.com request failing in the LPTT and verified
 the response is correct.