Add GC.preserve to extend the object lifecycle

[iblislin]

Try to fix the random failures in CI

e.g.

• https://github.com/JuliaML/LIBSVM.jl/pull/77/checks?check_run_id=2807830407

[codecov-commenter]

Codecov Report

Merging #79 (96d6d1a) into master (b018d8e) will increase coverage by 0.20%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #79      +/-   ##
==========================================
+ Coverage   84.54%   84.75%   +0.20%     
==========================================
  Files           5        5              
  Lines         220      223       +3     
==========================================
+ Hits          186      189       +3     
  Misses         34       34              

Impacted Files	Coverage Δ
src/LIBSVM.jl	93.93% <100.00%> (+0.11%)	⬆️

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