Constructor should copy only the desired chars

[MatthiasJentsch]

Description

If startIndex is not zero and length not value.length the last line in the constructor would throw an exception.

Motivation and Context

Found that during unit testing

How Has This Been Tested?

With this change the call StringBuilder sb = new StringBuilder("nanoFramework", 4, 5, 7) works correctly. Without this change the call throws an exception.

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)

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.

Signed-off-by: Matthias Jentsch info@matthias-jentsch.de

[nfbot]

Hi @MatthiasJentsch,

I'm nanoFramework bot.
Thank you for your contribution!

A human will be reviewing it shortly. 😉

[CLAassistant]

All committers have signed the CLA.

[josesimoes]

LGTM