fix playback freeze happening when seeking in http://wams.edgesuite.net/media/MPTExpressionData01/ElephantsDream_1080p24_IYUV_2ch.ism/manifest(format=m3u8-aapl)

this playlist contains several variant playlists.
each variant playlist contains only one #EXT-X-PROGRAM-DATE-TIME tag
this was not parsed correctly. parsing was assuming that each fragment contains such tag.
as the fragment selection logic is preferentially based on fragment date (if such date is available), wrong(old) fragment was selected when switching level.
this was breaking the playback