Fix SIGTRAP in Striped::Get with libc++ bounds checks

[smissingham]

Summary

Fix a macOS SIGTRAP seen during DB reopen after forced process termination.

The trap occurs in libc++ checked unique_ptr<T[]>::operator[] on the Striped::Get path used by TableCache::FindTable.

Root cause

Striped::Get used data_[index], which calls std::unique_ptr<T[]>::operator[].

Change

In util/mutexlock.h, update Striped::Get to:

• add defensive assert(index < stripe_count_) for explicit debug-time index invariant checking
• use *(data_.get() + index) instead of data_[index] to avoid the libc++ checked unique_ptr<T[]>::operator[] trap path

Validation

• Baseline reproduces with Clang/libc++ 21.1.8 + Snappy 1.2.2 + Zstd 1.5.7 (sigtrap after first kill cycle).
• With this patch, repeated runs on the same stack show sigtrap=0 (killed only).
• Cross-check with Clang/libc++ 16.0.6 + Snappy 1.2.1 + Zstd 1.5.6 remains non-repro (killed only).

References

• SurrealDB issue context: Frequent SIGTRAP with Surreal v3 rocksdb on MacOS surrealdb/surrealdb#6953
• Minimal repro repository: https://github.com/smissingham/rocksdb-cpp-sigtrap-repro
• Repro CI workflow runs: https://github.com/smissingham/rocksdb-cpp-sigtrap-repro/actions/workflows/repro.yml

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