Latent memory leaks in KEM_KEY setter functions

[justsmth]

Description of changes:

KEM_KEY_set_raw_public_key, KEM_KEY_set_raw_secret_key, and KEM_KEY_set_raw_key overwrite key->public_key and/or key->secret_key without first freeing any existing allocation. If these functions were ever called on a KEM_KEY that already had key material set, the previous buffers would be leaked.

• Current call sites are not affected because they always operate on freshly zero-allocated keys (via KEM_KEY_new), but the functions themselves are unsafe if reused in other contexts.

This change adds OPENSSL_free calls before each pointer is overwritten, consistent with how KEM_KEY_init already calls KEM_KEY_clear before re-initializing.

Call-outs:

This is a hardening fix. The added OPENSSL_free calls are no-ops in all current call sites (the pointers are always NULL), so there is no change in observable behavior today.

Testing:

Existing tests cover these code paths. No new tests required since the fix does not alter behavior for current callers.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.36%. Comparing base (0628190) to head (53f9b2f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3041   +/-   ##
=======================================
  Coverage   78.35%   78.36%           
=======================================
  Files         689      689           
  Lines      121010   121014    +4     
  Branches    16992    16991    -1     
=======================================
+ Hits        94813    94827   +14     
+ Misses      25302    25292   -10     
  Partials      895      895           

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