always clear reference to closed reader/writer

[beasteers]

Description of change

Fixes #2065 - makes sure we always discard closed/closing writers

Pull Request check-list

• Does $ tox pass with this change (including linting)? (tox doesn't pass on master and this doesn't attempt to fix any of those issues)
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?

NOTE: these things are not required to open a PR and can be done
afterwards / while the PR is open.

[codecov-commenter]

Codecov Report

Merging #2077 (deb281b) into master (ef4caf5) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2077      +/-   ##
==========================================
+ Coverage   91.20%   91.23%   +0.03%     
==========================================
  Files         100      100              
  Lines       21319    21361      +42     
==========================================
+ Hits        19443    19489      +46     
+ Misses       1876     1872       -4     

Impacted Files	Coverage Δ
redis/asyncio/connection.py	82.75% <100.00%> (+0.11%)	⬆️
tests/test_search.py	99.17% <0.00%> (+0.02%)	⬆️
redis/commands/cluster.py	93.57% <0.00%> (+0.09%)	⬆️
tests/test_cluster.py	97.54% <0.00%> (+0.12%)	⬆️
redis/asyncio/client.py	90.63% <0.00%> (+0.17%)	⬆️
redis/commands/search/query.py	93.33% <0.00%> (+0.22%)	⬆️

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[dvora-h]

@beasteers Awesome, thank you for reporting this bug and contribute its fix!