Fix parsing byterange attribute in HlsParser.
Fix parsing byterange attribute in HlsParser.

The BYTERANGE attribute on an hls playlist can either specify
start of the byterange and length or length only. If start
of the range is not specified, it means that the segment starts
right after the previous one, which means (prevSegment.endByte +
1). We used to have it as prevSegment.endByte instead.

Closes #925.

Change-Id: I207ed73dead7cba2c93587a00a519de5b5164f03

shaka-project/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,TheModMaker/shaka-player,vimond/shaka-player,shaka-project/shaka-player,TheModMaker/shaka-player,tvoli/shaka-player,vimond/shaka-player,TheModMaker/shaka-player,tvoli/shaka-player,tvoli/shaka-player,vimond/shaka-player,TheModMaker/shaka-player