ffserver: Calculate cur_pts using first_pts as origin instead of ist->start_time

This fixes cases where ist->start_time wasn't initialized. This also makes
cur_pts calculated against the same origin for all streams.

Originally committed as revision 23450 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffserver.c

@@ -2368,8 +2368,7 @@ static int http_prepare_data(HTTPContext *c)
                     if (c->is_packetized) {
                         /* compute send time and duration */
                         c->cur_pts = av_rescale_q(pkt.dts, ist->time_base, AV_TIME_BASE_Q);
-                        if (ist->start_time != AV_NOPTS_VALUE)
-                            c->cur_pts -= av_rescale_q(ist->start_time, ist->time_base, AV_TIME_BASE_Q);
+                        c->cur_pts -= c->first_pts;
                         c->cur_frame_duration = av_rescale_q(pkt.duration, ist->time_base, AV_TIME_BASE_Q);
                         /* find RTP context */
                         c->packet_stream_index = pkt.stream_index;