fix(treespec): fix potential segmentation fault for pickling support

[XuehaiPan]

Description

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Motivation and Context

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You can use the syntax close #15213 if this solves the issue #15213

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• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist

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• I have updated the documentation accordingly.
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (9a613a7) to head (34e20ee).
Report is 21 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #143   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines          977       988   +11     
=========================================
+ Hits           977       988   +11     

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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