pgwire: adds bounds check for FormatCodes

[rafiss]

fixes #103629

Now we check the bounds when looking for the format code to use.

Getting an out of bounds index is not expected, so we use an assertion error. This is less disruptive than a panic that crashes the node.

Release note: None

[cockroach-teamcity]

This change is 

[yuzefovich]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @rafiss)

[rafiss]

tftr!

bors r+

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from dd268d6 to blathers/backport-release-22.2-103645: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 22.2.x failed. See errors above.

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error setting reviewers, but backport branch blathers/backport-release-23.1-103645 is ready: POST https://api.github.com/repos/cockroachdb/cockroach/pulls/103696/requested_reviewers: 422 Reviews may only be requested from collaborators. One or more of the teams you specified is not a collaborator of the cockroachdb/cockroach repository. []

Backport to branch 23.1.x failed. See errors above.

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