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).
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Detailed description

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Test plan

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Closing issues

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[notxvilka]

In this case might make more sense to remove the asserts then.

P.S. Please use more descriptive titles for such PRs. I always have to edit the message when merging.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.69%. Comparing base (d72bc99) to head (c28c4eb).
⚠️ 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%)	⬆️

... and 115 files with indirect coverage changes

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