release-21.1: colexec: fix CASE operator a bit

[yuzefovich]

Backport 1/1 commits from #67757.

/cc @cockroachdb/release

Release justification: extremely low risk fix to an edge case bug that was hit
a few times in the wild.

---

Whenever we're updating the length on the batch containing bytes-like
vectors, we are updating those vectors to have non-decreasing offsets.
In case the batch has a selection vector set, we're using the largest
index in the selection to update the offsets. This logic relies on the
assumption that the selection vector is set on the batch before
setting the length which wasn't the case in a couple of places.

Note that there is no regression test because I cannot quite pin down
the exact conditions for the bug to occur (we need a bytes-like vector
in the batch but also such a selection vector that has garbage values
beyond the current length of the batch).

Fixes: #67744.

Release note (bug fix): Fixed very rare unexpected error from the
vectorized engine (index out of bounds) when evaluating CASE operator.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• 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.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[michae2]

Reviewed 3 of 3 files at r1.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @yuzefovich)