Fix frollapply to handle n > len case

[jangorecki]

closes #7646

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.02%. Comparing base (67e7840) to head (301409b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7650   +/-   ##
=======================================
  Coverage   99.02%   99.02%           
=======================================
  Files          87       87           
  Lines       16897    16900    +3     
=======================================
+ Hits        16733    16736    +3     
  Misses        164      164           

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[github-actions]

• HEAD=frollapply_n_lenx stopped early for fread(select=list(Date='date')) improved in #6107
  

Generated via commit 301409b

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	2 minutes and 38 seconds
Installing different package versions	21 seconds
Running and plotting the test cases	3 minutes and 29 seconds

[MichaelChirico]

Does it warrant a patch release?

[jangorecki]

No, function is said to be experimental so I think we can postpone 1.18.4 till CRAN will require us to patch something. Then this change should be included. Unless there will be many reports about it...