"invalid CSUP header" when reading CSUP that had been written after reading via fjson

[philrz]

$ curl -s -O https://data.gharchive.org/2023-02-08-0.json.gz &&
  gunzip 2023-02-08-0.json.gz &&
  super -version &&
  super -dynamic -vam -i fjson -f csup -o 2023-02-08-0.csup 2023-02-08-0.json &&
  super -dynamic -vam -i csup -c 'count()' 2023-02-08-0.csup

Version: v0.3.0-154-g6db8c25a3

2023-02-08-0.csup: invalid CSUP header

Details

Repro is with super commit 6db8c25, which is associated with the merge of the changes in #6982.

This problem did not occur at commit d25afc9 right before the changes merged from #6982.

$ super -version &&
  super -dynamic -vam -i fjson -f csup -o 2023-02-08-0.csup 2023-02-08-0.json &&
  super -dynamic -vam -i csup -c 'count()' 2023-02-08-0.csup

Version: v0.3.0-153-gd25afc96e

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