What does this PR do?:

Defensive hardening of the JVMTI sampled-object-alloc path in Profiler::recordJVMTISample:

1. After the per-shard tryLock succeeds, load _calltrace_buffer[lock_index] into a local and drop the sample if it is nullptr instead of dereferencing it (ddprof-lib/src/main/cpp/profiler.cpp:457-466).
2. In Profiler::start(), serialise buffer reallocation against signal-handler readers by allocating the replacement first, then swapping the slot under the matching _locks[i], and only freeing the old buffer once the lock is released (ddprof-lib/src/main/cpp/profiler.cpp:1115-1133). A calloc failure no longer leaves the slot pointing at freed memory.

Motivation:

Single observed SIGSEGV null-deref crash reported — Profiler::recordJVMTISample called from JvmtiExport::post_sampled_object_alloc, fault address 0x0, JDK 25.0.3 / amd64 / Linux, 1 event / 4 days / 1 service.

The data is too thin to confirm a single root cause — these are best-effort defensive measures, not a verified fix. They close the cheapest plausible window without changing hot-path behaviour or adding any allocations.

Triage notes posted on the JIRA ticket.

Triage summary

Walking the function for derefs reachable from the top frame, the closest match to a fault at 0x0 is _calltrace_buffer[lock_index]->_asgct_frames at profiler.cpp:461-462 (now 465-466): _asgct_frames is a union member at offset 0 of CallTraceBuffer, so a null _calltrace_buffer[lock_index] produces frames == nullptr, and the subsequent GetStackTrace(..., jvmti_frames, ...) write traps at 0x0.

For the slot to be observed null while ObjectSampler::_active = true, the most plausible window is Profiler::start() reallocation at profiler.cpp:1106-1114: free(_calltrace_buffer[i]) followed by calloc, with no per-shard lock held by start and no nulling between the two calls. A tryLock-based reader (the JVMTI sample path) does not synchronise against this writer, and a calloc failure path additionally leaves the slot pointing at freed memory.

VM::jvmti() and Profiler::instance() are statics that cannot become null at runtime; FlightRecorder::recordEvent already null-checks _rec under a shared lock. Inlining of Recording::recordAllocation writes into the same top frame remains a theoretical possibility but no obvious unguarded null was found there on inspection.

Additional Notes:

• Both changes are local and allocation-free on the hot path. The reader path adds one extra load and one branch (predictable: the slot is non-null in steady state). The writer adds a per-shard lock acquisition during Profiler::start() only.
• The reader uses a plain load: the writer publishes the pointer with the spin-lock's release barrier, and the reader's tryLock provides the matching acquire barrier.
• The change does not touch the other two consumers of _calltrace_buffer[lock_index] (recordSample at line ~559, recordExternalSample at line ~642). The writer-side serialisation under _locks[i] benefits them as well, but their explicit null-check is intentionally out of scope here.
• No new tests: the failure mode is a narrow start/stop/realloc race that cannot be exercised reliably from the existing gtests without introducing test-only hooks. Existing :ddprof-lib:gtestDebug suite still passes locally.

How to test the change?:

• :ddprof-lib:compileRelease succeeds locally — verified
• :ddprof-lib:gtestDebug passes locally — verified (8/8 in UtilsTest)
• CI green on Linux x86_64 / aarch64 and musl variants
• No regression on existing ObjectSampler JVMTI tests

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.
• JIRA: PROF-14679

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