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 (281ab78).

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 slower P<0.001 for DT[by,verbose=TRUE] improved in #6296
  

Generated via commit 281ab78

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

Task	Duration
R setup and installing dependencies	5 minutes and 18 seconds
Installing different package versions	11 minutes and 1 seconds
Running and plotting the test cases	4 minutes and 1 seconds