fix: copy ticket key before use to avoid concurrent rotation UAF

[jouho]

Goal

Fix a use-after-free in the session ticket encrypt/decrypt paths triggered by concurrent ticket key rotation.

Why

The lookup functions return a raw pointer into the shared, unlocked config->ticket_keys array, and callers hold it across crypto operations. A concurrent s2n_config_add_ticket_crypto_key() can realloc or shift that array, invalidating the pointer. This is an expected usage pattern. The docs explicitly permit rotating keys on a live config. Result: likely a crash, worst case a ticket encrypted with stale key material.

How

Copy the key onto the stack immediately after lookup, before any crypto operations, so no pointer into the shared array survives. Applied at all four call sites (s2n_store_to_cache, s2n_resume_decrypt_session, and the TLS1.2/1.3 NST write paths).

Callouts

• Closes the memory-safety window without adding locks, consistent with s2n's lock-free design. A benign torn read during the copy is still possible but only degrades to a full-handshake fallback.
• Lookup functions still return raw pointers, so callers must remember to copy. Removing that footgun is a possible follow-up.

Testing

Existing session ticket and resumption tests continue to pass.

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