elf.c: Check e_shstrndx against sections' length, not capacity

[kazarmy]

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).

Detailed description

In elf.c, this pr checks e_shstrndx against sections' length and not capacity since the former makes more sense, and sections' length should always be less than or equal its capacity so this is a tighter bound. The original check against capacity was done by rz_vector_index_ptr()'s precondition assertion.

This is a cherry-pick from #5102.

Test plan

The change makes sense. All builds are green.

Closing issues

...

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 43.91%. Comparing base (9f70aa1) to head (3af7cfb).
Report is 2 commits behind head on dev.

Files with missing lines	Patch %	Lines
librz/bin/format/elf/elf.c	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
librz/bin/format/elf/elf.c	72.19% <33.33%> (ø)

... and 12 files with indirect coverage changes

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