vindexes: fix pooled collator buffer memory leak

[brendar]

Description

Fixes a memory leak in unicode_loose_* vindexes by resetting the buffer before returning the collator to the pool.

This issue impacts v18 and below so I think it should be backported to v17 and v16 as well. As noted in the linked issue, the problem does not exist on main.

Related Issue(s)

Fixes #14587

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

n/a

[vitess-bot]

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