[playlists] fix concat playlist build with Non-monotonic DTS

[EvanBldy]

Problem

• Sometimes we have problems with concat playlists due to non-monotonic DTS in ffmpeg (see here : https://stackoverflow.com/questions/53021266/non-monotonous-dts-in-output-stream-previous-current-changing-to-this-may-result).

Solution

• Use a select instead.