fix: `kstrsub` function array wrong memory access (stack smashing)

To solve an error in the kstrsub function of the kstring module, this patch is being submitted. The memory access done in dest[start + len] = 0; is not correct, as the wanted substring is len long; thus, it is only needed to terminate the selected substring in dest after the amount of characters specified through the len argument.

Description

Please, include a summary of the changes and which issue(s) is/are fixed. Also include relevant motivation and context. Lastly, list any dependencies that are required for this change.

While predefining unit tests for kstring module (#23), it spit out the error: *** stack smashing detected ***: terminated. This message typically indicates a buffer overflow. This happens when you write more data to a block of memory (like an array) than it can hold; basically, when a program uses more stack memory than it's allowed to.

The issue is in the line containing: dest[start + len] = 0;. Here, it's trying to set the character at the index start + len to null, i.e. mark the string as terminated at that specific point. However, the specific substring wanted to obtain is exactly len chars long, so there is no point in terminating the dest array start more chars than it needs: appart from being innecessary, if the dest array is less than start + len long (perfectly feasible as the wanted substring to hold there is only len chars long), it will try to write past the end of dest, causing a buffer overflow.

Issues:

N/A

Motivation and context:
Detected wrong behavior while working on #23

Dependencies:
N/A

Development and testing equipment

Development PC specs:

CPU: R7 3800X 4.0GHz @ 1.05V
RAM: 96GB DDR4 2400MT/s
GPU: RTX 2060 SUPER
OS: GNU/Linux (Gentoo)
Glibc version: 2.37
GCC version: 13.2.1
Vulkan version: 1.3.261

Please, describe the tests that you ran to verify your changes. Provide instructions so it can be reproducible. Also list any relevant details for your test configuration.
Preliminary version of unit tests implemented in #23; testing the function when obtaining a substring from the start, mid, and end of a predefined string.

Type of change

[x] Bug fix (non-breaking change which fixes one or multiple issues).
[ ] New feature (non-breaking change which adds functionality).
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

[x] I've read and comply with the contributing guidelines.
[ ] I've tested my changes for new features and regressions on all supported operating systems.
[ ] I've made corresponding changes to the documentation.
[ ] I've checked my changes for any broken behaviour and/or any unjustified performance hits.
[x] My changes generate no new warnings.
[ ] I've added tests that prove my fix is effective or that my feature works.
[x] New and existing tests pass locally with my changes.

iWas-Coder / wge