fix: fix s2n_io_pair_close_one_end

[boquan-fang]

Resolved issues:

Partially Solve #4005.

Our current implementation for s2n_io_pair_close_one_end has problems:

• It might attempt to close a already closed fd, and failed the s2n_io_pair_close function.
• It might accidentally close a valid fd, that is reused by one closed end of io_pair.

Description of changes:

• Add checking logics for s2n_io_pair_close_one_end().
  • Only proceed to close a fd if the fd is not previously closed.
  • Define S2N_CLOSED_FD to be -1.
  • Set closed fd to S2N_CLOSED_FD.

Call-outs:

• This PR is related to PR4833. It is to solve this comment.

Testing:

• It passes all unit tests.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.