dogroups: don't access columns beyond end of table

[aitap]

Closes #7486

Previously, dogroups() could try to read elements after the (resized) end of over-allocated data.table list, expecting them to be NULL. This didn't crash in practice, but (as of r89198) is explicitly checked for (and disallowed):

  > DT[, grp := .GRP, by=x]                 # add a group counter column
  Error in `[.data.table`(DT, , `:=`(grp, .GRP), by = x) : 
    attempt access index 3/3 in VECTOR_ELT
  Calls: [ -> [.data.table

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.04%. Comparing base (e35dec5) to head (20154a3).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7485   +/-   ##
=======================================
  Coverage   99.04%   99.04%           
=======================================
  Files          87       87           
  Lines       16671    16671           
=======================================
  Hits        16511    16511           
  Misses        160      160           

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[github-actions]

• HEAD=vector_elt_overflow stopped early for DT[,.SD] improved in #4501
  

Generated via commit 20154a3

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

Task	Duration
R setup and installing dependencies	2 minutes and 52 seconds
Installing different package versions	44 seconds
Running and plotting the test cases	5 minutes and 26 seconds

[ben-schwen]

LGTM TY. I also added a NEWS. @TysonStanley this will most definitely need to be picked for resubmit :/