fftools/ffmpeg: move initializing next_[pd]ts to add_input_streams()

They are initialized to constants, so it makes most sense to do it as
soon as possible.

fftools/ffmpeg.c

@@ -2848,9 +2848,6 @@ static int init_input_stream(InputStream *ist, char *error, int error_len)
         assert_avoptions(ist->decoder_opts);
     }
 
-    ist->next_pts = AV_NOPTS_VALUE;
-    ist->next_dts = AV_NOPTS_VALUE;
-
     return 0;
 }
 

fftools/ffmpeg_demux.c

@@ -675,6 +675,9 @@ static void add_input_streams(const OptionsContext *o, Demuxer *d)
         st->discard  = AVDISCARD_ALL;
         ist->nb_samples = 0;
         ist->first_dts = AV_NOPTS_VALUE;
+        ist->next_pts  = AV_NOPTS_VALUE;
+        ist->next_dts  = AV_NOPTS_VALUE;
+
         ds->min_pts = INT64_MAX;
         ds->max_pts = INT64_MIN;