fftools/ffmpeg: stop using av_stream_get_end_pts()

It retrieves the muxer's internal timestamp with under-defined
semantics. Continuing to use this value would also require
synchronization once the muxer is moved to a separate thread.

Replace the value with last_mux_dts.

fftools/ffmpeg.c

@@ -1669,9 +1669,8 @@ static void print_report(int is_last_report, int64_t timer_start, int64_t cur_ti
             vid = 1;
         }
         /* compute min output value */
-        if (av_stream_get_end_pts(ost->st) != AV_NOPTS_VALUE) {
-            pts = FFMAX(pts, av_rescale_q(av_stream_get_end_pts(ost->st),
-                                          ost->st->time_base, AV_TIME_BASE_Q));
+        if (ost->last_mux_dts != AV_NOPTS_VALUE) {
+            pts = FFMAX(pts, ost->last_mux_dts);
             if (copy_ts) {
                 if (copy_ts_first_pts == AV_NOPTS_VALUE && pts > 1)
                     copy_ts_first_pts = pts;