Fix access beyond array boundaries in SpanByte

[gukoff]

Description

Fix array access similarly to how it's done in nanoframework/nanoFramework.IoT.Device#29

Motivation and Context

• Fixes a bug

How Has This Been Tested?

Not tested.

Screenshots

Types of changes

• Improvement (non-breaking change that improves a feature, code or algorithm)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Config and build (change in the configuration and build system, has no impact on code or features)
• Dependencies (update dependencies and changes associated, has no impact on code or features)
• Unit Tests (work on Unit Tests, has no impact on code or features)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[gukoff]

I think there are no tests for SpanByte in the repo. Should we add them in this PR too?

[josesimoes]

Yes please! Let's have unit tests for this.

[Ellerbach]

Thanks for the submission. And spotting the potential issue

[gukoff]

My VS doesn't recognize the tests here, I think because there are special guidelines. I'll see if following them fixes the issue, and then look into tests.