Fixed ODIDMediaEngine handling of TimeSeekRange requests with npt range start values beyond the last keyframe

The offsets_covering_time_range() function was throwing an exception if it
 didn't find a keyframe with a time preceding the requested time range.
 Changed it to only throw an exception if the start time was greater than
 the total duration and use the last keyframe as the range start if the
 requested start was beyond the last keyframe.

Ran TimeSeekRange tests with modified index file to recreate the condition
 being seen in the DLNA CTT test. Confirmed the correct results with
 this fix.