release-23.2: execbuilder: fix a silly bug around finding the most recent full stat

[yuzefovich]

Backport 1/1 commits from #148329.

/cc @cockroachdb/release

---

This commit fixes a bug that was added in 3bc0992. In short, in a complicated logical expression we had v1 && v2 || v3 when we wanted v1 && (v2 || v3). This allowed us to hit an index of bounds when evaluating v3, which in this concrete case could mean that we iterated over all stats and didn't find any full ones.

There are two other places with similar conditionals, and they have the correct usage of parenthesis, so I didn't change the structure overall.

Also the bug seems quite difficult to hit, so I omitted the release note.

Fixes: #148316.

Release note: None

Release justification: critical bug fix: we've seen this bug result in a failure to take a backup (#148316) since it can error out an internal query used by the backup (another example is #148414 with read-role-with-memberships internal query). In the extreme, failure to backup data can result in a data loss.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
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  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
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  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[michae2]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @yuzefovich)