server: fix panic due to regression in #887

[aler9]

This happened when writing a TCP packet to a conn after a read error.

[codecov]

Codecov Report

❌ Patch coverage is 83.01887% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.15%. Comparing base (ead4471) to head (26816c6).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
server_conn_reader.go	82.60%	4 Missing ⚠️
client_reader.go	89.47%	2 Missing ⚠️
server.go	0.00%	1 Missing and 1 partial ⚠️
client.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #892   +/-   ##
=======================================
  Coverage   81.15%   81.15%           
=======================================
  Files         131      131           
  Lines       14733    14751   +18     
=======================================
+ Hits        11956    11971   +15     
- Misses       2246     2248    +2     
- Partials      531      532    +1     

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