Fix FileStream memory leak

[alberk8]

Description

1. Remove the vfsPath platform_malloc.
2. Remove check for vfsPath == NULL;

Motivation and Context

• Leak native memory very time a file is opened or written to.
• Fixes SDCard native library leaking memory Home#895

How Has This Been Tested?

loop 100 times.
https://github.com/alberk8/nanoFramework-SDCard-Test/tree/main/nanoFrameworkSDCard

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)
• Documentation (changes or updates in the documentation, 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.

[nfbot]

@alberk8 there are issues with the code style on the source files.
A PR was submitted with the code style fixes. Please click alberk8#2, review the changes if you want and merge it.

Make sure to follow the project code style. Check the details here on how it works and the tools required to help you with that.

[josesimoes]

Nice job chasing this down!
I've added a missing check for succesfulll malloc and comments stressing the need to free memory by the caller.

It's not perfect, but will get the job done.

[nfbot]

@alberk8 there are issues with the code style on the source files.
A PR was submitted with the code style fixes. Please click alberk8#3, review the changes if you want and merge it.

Make sure to follow the project code style. Check the details here on how it works and the tools required to help you with that.