avformat/mpegtsenc: make first_pcr sync with the first valid dts

now first_pts assume dts will start from zero, if it's not true(copyts is enable),
too many null packet will be inserted for cbr output.

Please test with below command, you'll get huge test.ts without the patch:
./ffmpeg -y -copyts -i ../fate-suite/mpegts/loewe.ts  -c:v libx264 -x264opts \
   nal-hrd=cbr:force-cfr=1 -b:v 3500k -minrate 3500k -maxrate 3500k -bufsize \
   1000k  -c:a mp2 -muxrate 4500k  -vframes 1000 test.ts

Reviewed-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Limin Wang <lance.lmwang@gmail.com>

libavformat/mpegtsenc.c

@@ -81,6 +81,7 @@ typedef struct MpegTSWrite {
     int64_t pat_period; /* PAT/PMT period in PCR time base */
     int nb_services;
     int64_t first_pcr;
+    int first_dts_checked;
     int64_t next_pcr;
     int mux_rate; ///< set to 1 when VBR
     int pes_payload_size;
@@ -1688,6 +1689,11 @@ static int mpegts_write_packet_internal(AVFormatContext *s, AVPacket *pkt)
         stream_id = side_data[0];
 
     if (ts->copyts < 1) {
+        if (!ts->first_dts_checked && dts != AV_NOPTS_VALUE) {
+            ts->first_pcr += dts * 300;
+            ts->first_dts_checked = 1;
+        }
+
         if (pts != AV_NOPTS_VALUE)
             pts += delay;
         if (dts != AV_NOPTS_VALUE)