ffmpeg: Fix flush packet stream copy input timestamp handling

Since a7da134742, flush packets are passed
to process_input_packet() during stream copy.  This modifies the input
timestamp handling to ignore them - since they contain no data, timestamps
should not be affected.

fftools/ffmpeg.c

@@ -2699,7 +2699,7 @@ static int process_input_packet(InputStream *ist, const AVPacket *pkt, int no_eo
     }
 
     /* handle stream copy */
-    if (!ist->decoding_needed) {
+    if (!ist->decoding_needed && pkt) {
         ist->dts = ist->next_dts;
         switch (ist->dec_ctx->codec_type) {
         case AVMEDIA_TYPE_AUDIO:
@@ -2712,7 +2712,7 @@ static int process_input_packet(InputStream *ist, const AVPacket *pkt, int no_eo
                 AVRational time_base_q = AV_TIME_BASE_Q;
                 int64_t next_dts = av_rescale_q(ist->next_dts, time_base_q, av_inv_q(ist->framerate));
                 ist->next_dts = av_rescale_q(next_dts + 1, av_inv_q(ist->framerate), time_base_q);
-            } else if (pkt && pkt->duration) {
+            } else if (pkt->duration) {
                 ist->next_dts += av_rescale_q(pkt->duration, ist->st->time_base, AV_TIME_BASE_Q);
             } else if(ist->dec_ctx->framerate.num != 0) {
                 int ticks= av_stream_get_parser(ist->st) ? av_stream_get_parser(ist->st)->repeat_pict + 1 : ist->dec_ctx->ticks_per_frame;