increment initial_prog_date_time when removing old segments

Fixes https://trac.ffmpeg.org/ticket/5940#ticketReviewed-by: Michael Niedermayer <&lt;michael@niedermayer.cc&gt;>
Signed-off-by: Robert Nagy <ronag89@gmail.com>
Signed-off-by: Steven Liu <lingjiujianke@gmail.com>

libavformat/hlsenc.c

@@ -183,6 +183,7 @@ static int hls_delete_old_segments(HLSContext *hls) {
     segment = hls->old_segments;
     while (segment) {
         playlist_duration -= segment->duration;
+        hls->initial_prog_date_time += segment->duration;
         previous_segment = segment;
         segment = previous_segment->next;
         if (playlist_duration <= -previous_segment->duration) {