fix UB when opening an empty sparse layer

[youhangwang]

What this PR does / why we need it:

open_file_rw() uses &mappings[0] even when mappings may be empty. An empty sparse file produces zero SegmentMapping entries from create_mappings(), making &mappings[0] undefined behavior.

Use mappings.data() instead, which is well-defined for an empty vector and allows the existing zero-length case to be handled safely.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[Copilot]

🟡 Changes recommended

A small regression test covering “open empty sparse RW layer (0 mappings)” should be added/strengthened to prevent this UB from reappearing unnoticed.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Fixes undefined behavior in the sparse RW open path by avoiding &mappings[0] when the mappings vector is empty, ensuring empty sparse layers can be opened safely.

Changes:

• Replace &mappings[0] with mappings.data() when constructing the in-memory index for sparse RW layers.

File summaries

File	Description
src/overlaybd/lsmt/file.cpp	Avoids UB when passing an empty std::vector<SegmentMapping> to create_memory_index0() during sparse RW open.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite

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