lavf/utils: fix start_time/duration dump if it is AV_NOPTS_VALUE

e,g: the command:
ffprobe -show_format -i fate-suite/aac/foo.aac -loglevel 99 will
dump the trace message as follow when start_time is AV_NOPTS_VALUE
[aac @ 0x55bf8e1f3dc0] stream 0: start_time: -326791809695.818 duration: 2.174
[aac @ 0x55bf8e1f3dc0] format: start_time: -9223372036854.775 duration: 2.174 bitrate=120 kb/s

after this fix, will dump the start_time with "NOPTS".
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

libavformat/utils.c

@@ -2984,16 +2984,16 @@ static void estimate_timings(AVFormatContext *ic, int64_t old_offset)
         for (i = 0; i < ic->nb_streams; i++) {
             st = ic->streams[i];
             if (st->time_base.den)
-                av_log(ic, AV_LOG_TRACE, "stream %d: start_time: %0.3f duration: %0.3f\n", i,
-                       (double) st->start_time * av_q2d(st->time_base),
-                       (double) st->duration   * av_q2d(st->time_base));
+                av_log(ic, AV_LOG_TRACE, "stream %d: start_time: %s duration: %s\n", i,
+                       av_ts2timestr(st->start_time, &st->time_base),
+                       av_ts2timestr(st->duration, &st->time_base));
         }
         av_log(ic, AV_LOG_TRACE,
-                "format: start_time: %0.3f duration: %0.3f (estimate from %s) bitrate=%"PRId64" kb/s\n",
-                (double) ic->start_time / AV_TIME_BASE,
-                (double) ic->duration   / AV_TIME_BASE,
-                duration_estimate_name(ic->duration_estimation_method),
-                (int64_t)ic->bit_rate / 1000);
+               "format: start_time: %s duration: %s (estimate from %s) bitrate=%"PRId64" kb/s\n",
+               av_ts2timestr(ic->start_time, &AV_TIME_BASE_Q),
+               av_ts2timestr(ic->duration, &AV_TIME_BASE_Q),
+               duration_estimate_name(ic->duration_estimation_method),
+               (int64_t)ic->bit_rate / 1000);
     }
 }