mysql: remove more allocations from parseOKPacket

[vmg]

Description

ooops! I actually forgot to push the second commit from #15067 before that PR was merged. My bad!

This commit removes the second allocation as described in the PR description for the previous PR. With this change, the parseOKPacket API is actually zero-allocation.

Related Issue(s)

• mysql: do not allocate in parseOKPacket #15067
• performance: parseOKPacket should not allocate #15071

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (c156ca2) 47.70% compared to head (e3faf2c) 47.71%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
go/mysql/conn.go	14.28%	6 Missing ⚠️
go/mysql/query.go	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15082   +/-   ##
=======================================
  Coverage   47.70%   47.71%           
=======================================
  Files        1155     1155           
  Lines      240231   240218   -13     
=======================================
+ Hits       114610   114612    +2     
+ Misses     117018   117006   -12     
+ Partials     8603     8600    -3     

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