Null-guard EVP_AEAD_CTX_cleanup for fork safety with shared memory

[WillChilds-Klein]

Issues:

Resolves #P425555771

Description of changes:

Summary

Applications that use fork() with MAP_SHARED memory for TLS connection state can trigger a SIGSEGV in EVP_AEAD_CTX_cleanup. This occurs because DEFINE_METHOD_FUNCTION lazily initializes AEAD vtables via CRYPTO_once into .bss storage. After fork(), each process has an independent .bss copy. If a child process initializes the vtable (by performing TLS) and stores an EVP_AEAD_CTX in shared memory, the parent process's vtable remains zeroed. When the parent calls EVP_AEAD_CTX_cleanup, it dereferences ctx->aead->cleanup
which is NULL, crashing the process.

Call-outs:

n/a

Testing:

Adds aead_fork_test, an isolated test binary that verifies EVP_AEAD_CTX_cleanup tolerates an uninitialized vtable after fork(). The test is a separate executable (like rand_isolated_test) because it requires that AEAD vtables are NOT initialized before the test runs. The FIPS power-on self-test and other tests in crypto_test trigger vtable initialization as a side effect.

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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.

[github-actions]

clang-tidy made some suggestions

[codecov-commenter]

Codecov Report

❌ Patch coverage is 56.25000% with 35 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.14%. Comparing base (e537abc) to head (7bb57a4).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
crypto/cipher_extra/aead_fork_test.cc	55.12%	31 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3280      +/-   ##
==========================================
- Coverage   78.39%   78.14%   -0.25%     
==========================================
  Files         693      694       +1     
  Lines      123806   123882      +76     
  Branches    17195    17197       +2     
==========================================
- Hits        97060    96813     -247     
- Misses      25826    26147     +321     
- Partials      920      922       +2     

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