Fix unresolved sticky class GC roots

[wilx]

Fixes #6357.

ROOT_STICKY_CLASS roots can refer to class object ids that are known from LOAD_CLASS records but are not represented as resolvable Instances by this parser. In the attached issue dump, the failing sticky-class roots are known loaded classes, but GCRoot.getInstance() returns null for them.

LOAD_CLASS records describe classes loaded by the VM: they provide the class object id and class name metadata. They do not by themselves provide the full heap object data that NetBeans uses to construct an Instance; that comes from matching heap dump records such as CLASS_DUMP. When a sticky-class root points at a LOAD_CLASS id without a matching resolvable instance entry, the parser can identify the loaded class metadata but cannot return an Instance, so the result is null.

HprofGCRoots.getGCRoot(Long) assumed every parsed GC root has a non-null Instance when building its instance lookup map. That made retained-size computation fail with a NullPointerException before it could finish walking nearest GC roots.

This change makes the GC-root lookup tolerate those unresolved roots. I also added a small synthetic HPROF regression that writes an unresolved sticky class root and verifies GC-root lookup handles it.

Validation performed:

• Reproduced the original NPE with the dump from issue NPE when parsing STICKY_CLASS tags in hprof heap dump #6357 before the fix.
• Verified the same getBiggestObjectsByRetainedSize(15) reproducer succeeds with the fixed profiler jar.
• Ran HeapSegmentTest: OK (3 tests).
• Ran broader heap unit classes; only the existing testHeapDumpLog golden-file assertions failed due to JVM system property output differences, unrelated to this change.

Assisted-by: OpenAI GPT-5 Codex

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