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>