Bug: addresses OSError related to "Bad file descriptor" messages that could occur after a socket has closed (SYN-7773)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.75%. Comparing base (5cd492e) to head (dd10c9e).

Files	Patch %	Lines
synapse/lib/link.py	93.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3831 +/- ## ========================================== - Coverage 96.82% 96.75% -0.07% ========================================== Files 241 241 Lines 53086 53089 +3 ========================================== - Hits 51399 51365 -34 - Misses 1687 1724 +37 ``` | [Flag](https://app.codecov.io/gh/vertexproject/synapse/pull/3831/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vertexproject) | Coverage Δ | | |---|---|---| | [linux](https://app.codecov.io/gh/vertexproject/synapse/pull/3831/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vertexproject) | `96.75% <93.33%> (+<0.01%)` | :arrow_up: | | [linux_replay](https://app.codecov.io/gh/vertexproject/synapse/pull/3831/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vertexproject) | `?` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vertexproject#carryforward-flags-in-the-pull-request-comment) to find out more.

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