fix: shouldn't use stack-pointer to avoid stackgrow problem

[AsterDY]

Reason

Since this place reference a pointer from the stack, once stackgrow happens after entering encoderTypedPointer, the pointer may be invalid and introduce potential NPE problem.

Fix

Malloc a new(unsafe.Pointer) obj on heap and store its pointer onto stack. Then write the value pointer vp onto the obj

Affect

Will introduce more malloc when encoding large structure. For our test sample Twitter, it will cause 6%~8% downgrades of performance

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8c71eb0) 78.57% compared to head (90bacc6) 78.60%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #515      +/-   ##
==========================================
+ Coverage   78.57%   78.60%   +0.03%     
==========================================
  Files          69       69              
  Lines       10714    10732      +18     
==========================================
+ Hits         8418     8436      +18     
  Misses       1930     1930              
  Partials      366      366              

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