Avoid out-of-bounds access in overlaps

[aitap]

When y has no rows, the data pointers of the columns of unique(y[,...]) are poisoned. Instead of trying to dereference them, pretend that all comparisons fail and fill the return value with NAs.

When there are no matches on the non-range columns, the index in from[i] may become 0. So uncomment the (from[i]>0) ? from[i] : 1 checks instead of trusting from[i] and accessing VECTOR_ELT(lookup, -1). I am especially interested in someone double-checking this part because there are other places where from[i] is not checked for being > 0. Are they reachable with from[i] == 0?

Fixes: #7597

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.00%. Comparing base (0216983) to head (9688a35).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7598   +/-   ##
=======================================
  Coverage   99.00%   99.00%           
=======================================
  Files          87       87           
  Lines       16893    16897    +4     
=======================================
+ Hits        16725    16729    +4     
  Misses        168      168           

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[github-actions]

• HEAD=fix-7597 slower P<0.001 for memrecycle regression fixed in #5463
  

Generated via commit 9688a35

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

Task	Duration
R setup and installing dependencies	5 minutes and 11 seconds
Installing different package versions	10 minutes and 56 seconds
Running and plotting the test cases	4 minutes and 8 seconds