Fix crash in the evalengine

[dbussink]

The allocation arena used here the wrong type to check for the size. The logic here was copied, but this case was missed.

We add a test here and fix the bug. It applies to both enums and sets.

Should be backported to all supported versions as this can lead to a panic.

Related Issue(s)

Fixes #17486

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• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
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[vitess-bot]

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