opt/exec/explain: gracefully handle decoding negative integers

[mgartner]

Fixes #133015

Release note: None

[cockroach-teamcity]

This change is 

[DrewKimball]

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

[mgartner]

TFTR!

bors r+

[craig]

Build succeeded:

• linux_arm64_build
• local_roachtest
• acceptance
• macos_amd64_build
• local_roachtest_fips
• linux_amd64_fips_build
• check_generated_code
• macos_arm64_build
• linux_amd64_build
• lint
• unit_tests
• docker_image_amd64
• examples_orms
• windows_build

[blathers-crl]

Based on the specified backports for this PR, I applied new labels to the following linked issue(s). Please adjust the labels as needed to match the branches actually affected by the issue(s), including adding any known older branches.

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Issue #133015: branch-release-23.1, branch-release-24.1, branch-release-24.2, branch-release-24.3.

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_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[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 799e9d4 to blathers/backport-release-23.1-133245: 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 23.1.x failed. See errors above.

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error creating merge commit from 799e9d4 to blathers/backport-release-23.2-133245: 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 23.2.x failed. See errors above.

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_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}