Dashdemux : adaptive stream REW problem

there are REW issues when playback fragmented MP4 files with mpd file.

issue No.1 : when reach first fragment during REW, playtime go to 0 sec directly.

solution : When the fragment info is updated by entering the first fragment of REW, set current timestamp to fragment duration.

issue No.2 : if fragment is changed during REW, video is stopped during few second and playtime increase.

solution : in case of REW and if sidx_position is None, set entry index to last entry index