Fix: set freed pointers to nullptr in destructors

[Besroy]

Set m_bytes and m_journal_entry to nullptr after manual deallocation to prevent potential double-free and use-after-free issues. This is a defensive programming practice that makes the code more robust against future changes and easier to debug.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 49.14%. Comparing base (1a0cef8) to head (1d6a1d7).
⚠️ Report is 306 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #849      +/-   ##
==========================================
- Coverage   56.51%   49.14%   -7.37%     
==========================================
  Files         108      110       +2     
  Lines       10300    11461    +1161     
  Branches     1402     5430    +4028     
==========================================
- Hits         5821     5633     -188     
+ Misses       3894     2195    -1699     
- Partials      585     3633    +3048     

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