setlevels(): avoid crash on missing factor values

[aitap]

Check the index for being NA_INTEGER before trying to access it.

Should this also protect from invalid factors? For example, this also currently crashes:

data.table::setattr(structure(integer(999), class = "factor", levels = "a"), "levels", c("b", "b"))

Should this go into 1.18.2?

Fixes: #7595

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.02%. Comparing base (d3d0019) to head (22e0f2e).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7596   +/-   ##
=======================================
  Coverage   99.02%   99.02%           
=======================================
  Files          87       87           
  Lines       16804    16806    +2     
=======================================
+ Hits        16640    16642    +2     
  Misses        164      164           

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[github-actions]

• HEAD=fix-setattr-crash slower P<0.001 for memrecycle regression fixed in #5463
  

Generated via commit 22e0f2e

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

Task	Duration
R setup and installing dependencies	3 minutes and 15 seconds
Installing different package versions	22 seconds
Running and plotting the test cases	4 minutes and 3 seconds

[aitap]

Test 2361.2 could also signal an error, similar to how base::`levels<-` fails (but doesn't crash) when given such a factor.

[MichaelChirico]

Should this go into 1.18.2?

I'd say yes, anytime we eliminate a crash it's important to include it ASAP

[MichaelChirico]

Thanks!