[.data.table: call setalloccol for non-selfrefok tables when removing columns

[aitap]

Before #7451, removing a column was always possible, because using SETLENGTH to reduce the length of a data.table was mostly unchecked (and didn't cause problems in practice, modulo the allocated memory count sometimes being wrong), even if it was not created by data.table code in the current R session. R_resizeVector now requires its argument to be resizable, and freshly unserialized data.tables don't pass the mark: ones created by older versions of data.table lack the growable bit, and ones created after #7451 latest version still have a zero TRUELENGTH.

Solution:

• call selfrefok even if not adding new columns
• if jval indicates removing a column, call setalloccol to prevent the error

Could the test for := removing a column be improved? (What about list columns containing NULLs?)

Fixes: #7488

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.04%. Comparing base (be67a4d) to head (8773401).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7492   +/-   ##
=======================================
  Coverage   99.04%   99.04%           
=======================================
  Files          87       87           
  Lines       16671    16678    +7     
=======================================
+ Hits        16511    16518    +7     
  Misses        160      160           

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[github-actions]

• HEAD=more-setalloccol stopped early for DT[,.SD] improved in #4501
• HEAD=more-setalloccol slower P<0.001 for isoweek improved in #7144
  

Generated via commit 8773401

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

Task	Duration
R setup and installing dependencies	5 minutes and 5 seconds
Installing different package versions	11 minutes and 15 seconds
Running and plotting the test cases	5 minutes and 25 seconds

[ben-schwen]

TYVM!

[ben-schwen]

@TysonStanley if still possible, we should squeeze this one into release, since otherwise we will get caught by revdeps

[TysonStanley]

Great! I'm using what is in master by tonight if that works for the team. @ben-schwen @aitap @jangorecki @MichaelChirico