Avoid getting start time / end time from null newSegment
Avoid getting start time / end time from null newSegment

If the newSegment is null, we should not call getStartTime() or
getEndTime() with it.

Change-Id: Ia1d548cabd33713e02f36176b2b9eab58ce54f94

shaka-project/shaka-player,tvoli/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player