[1.x] janus-pp-rec fails with faststart option when running on AV1 recordings

[corthiclem]

What version of Janus is this happening on?
v1.2.1

Have you tested a more recent version of Janus too?
No, v1.2.1 is the latest release as of today.

Was this working before?
Never tried before

Is there a gdb or libasan trace of the issue?
No

Additional context
I am trying to use the faststart option of janus-pp-rec with AV1 recordings to create .mp4 files but it appears to be broken somehow. I get the following error when running janus-pp-rec -t input.mjr output.mp4:

Unable to re-open  output file for shifting data

It seems to be related to #3037.

I'll try to create a pull request to fix the issue.