-use_wallclock_as_timestamps 1 not working with piped input

[Wes Castro]

Summary of the bug:
When piping a raw H264 stream to FFmpeg with -use_wallclock_as_timestamps 1, wallclock times are not used. If the pipe is opened for say 10 seconds, it's expected that the output's duration will be ~10 seconds. Instead it seems to use timing info from the input stream. I didn't test with other types of piped inputs

The issue was introduced in FFmpeg 6.1 and repros on the master branch.

How to reproduce:

ffmpeg -y -f lavfi -i testsrc=duration=1:size=1280x720:rate=30 -f h264 ~/1second.h264
(cat ~/1second.h264; sleep 10; cat ~/1second.h264) | ffmpeg  -use_wallclock_as_timestamps 1 -i pipe: -f mp4 ~/test.mp4 -y

Here are the durations reported by FFprobe when using various FFmpeg versions for the second line:

• FFmpeg 6.0: 11.5s
• FFmpeg 6.1: 2.0s
• FFmpeg 7.1: 2.0s
• master branch (08b1bffa49715a9615acc025dfbea252d8409e1f): 2.0s