refactor: call next for passthrough post

[shivamsouravjha]

Related Issue

• race condition while using callNext

Closes: #1466

Describe the changes you've made

• CustomConn uses ReadBytes which uses c.r.Read(p) which access the first element of array.
• Added mutex as to ensure no race condition occurs to avoid buffer read concurrently.
• Added check of eof
• update CI to ensure this case is tested as well

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, local variables)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Please let us know if any test cases are added

Please describe the tests(if any). Provide instructions how its affecting the coverage.

Describe if there is any unusual behaviour of your code(Write NA if there isn't)

A clear and concise description of it.

Checklist:

• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.

Screenshots (if any)

Original	Updated
original screenshot	updated screenshot

[gouravkrosx]

LGTM