Check for empty vector to prevent OOB reads and writes.

[Rot127]

Your checklist for this pull request

• [ ] I've read the guidelines for contributing to this repository.
• [ ] I made sure to follow the project's coding style.
• [ ] I've documented every RZ_API function and struct this PR changes.
• [ ] I've added tests that prove my changes are effective (required for changes to RZ_API).
• [ ] I've updated the Rizin book with the relevant information (if needed).
• [ ] I've used AI tools to generate fully or partially these code changes and I'm sure the changes are not copyrighted by somebody else.

Detailed description

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Test plan

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Closing issues

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[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 46.69%. Comparing base (d72bc99) to head (c28c4eb). :warning: Report is 50 commits behind head on dev.

Additional details and impacted files

Files with missing lines	Coverage Δ
librz/util/vector.c	78.52% <100.00%> (+0.42%)	:arrow_up:

... and 115 files with indirect coverage changes

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