avconv: better next_dts usage.

next_dts is used for estimating the dts of the next packet if it's
missing. Therefore, it makes no sense to set it from the pts of the last
decoded frame. Also it should be estimated from the current packet
duration/ticks_per_frame always, not only when a frame was successfully
decoded.

avconv.c

@@ -1955,17 +1955,8 @@ static int transcode_video(InputStream *ist, AVPacket *pkt, int *got_output, int
         /* no picture yet */
         return ret;
     }
-    ist->next_dts = decoded_frame->pts = guess_correct_pts(&ist->pts_ctx, decoded_frame->pkt_pts,
+    decoded_frame->pts = guess_correct_pts(&ist->pts_ctx, decoded_frame->pkt_pts,
                                            decoded_frame->pkt_dts);
-    if (pkt->duration)
-        ist->next_dts += av_rescale_q(pkt->duration, ist->st->time_base, AV_TIME_BASE_Q);
-    else if (ist->st->codec->time_base.num != 0) {
-        int ticks      = ist->st->parser ? ist->st->parser->repeat_pict + 1 :
-                                           ist->st->codec->ticks_per_frame;
-        ist->next_dts += ((int64_t)AV_TIME_BASE *
-                          ist->st->codec->time_base.num * ticks) /
-                          ist->st->codec->time_base.den;
-    }
     pkt->size = 0;
     pre_process_video_frame(ist, (AVPicture *)decoded_frame, &buffer_to_free);
 
@@ -2128,6 +2119,13 @@ static int output_packet(InputStream *ist,
             break;
         case AVMEDIA_TYPE_VIDEO:
             ret = transcode_video    (ist, &avpkt, &got_output, &pkt_pts);
+            if (avpkt.duration)
+                ist->next_dts += av_rescale_q(avpkt.duration, ist->st->time_base, AV_TIME_BASE_Q);
+            else if (ist->st->codec->time_base.num != 0) {
+                int ticks      = ist->st->parser ? ist->st->parser->repeat_pict + 1 :
+                                                   ist->st->codec->ticks_per_frame;
+                ist->next_dts += av_rescale_q(ticks, ist->st->codec->time_base, AV_TIME_BASE_Q);
+            }
             break;
         case AVMEDIA_TYPE_SUBTITLE:
             ret = transcode_subtitles(ist, &avpkt, &got_output);