Avoid sending messages on closed or closing Websocket

[philippjfr]

Fixes #3113

[codecov]

Codecov Report

Merging #3115 (576eba8) into master (77882db) will decrease coverage by 0.02%.
The diff coverage is 4.76%.

@@            Coverage Diff             @@
##           master    #3115      +/-   ##
==========================================
- Coverage   83.10%   83.07%   -0.03%     
==========================================
  Files         192      192              
  Lines       25460    25463       +3     
==========================================
- Hits        21159    21154       -5     
- Misses       4301     4309       +8     

Impacted Files	Coverage Δ
panel/io/server.py	71.94% <0.00%> (-0.93%)	⬇️
panel/reactive.py	76.28% <ø> (-0.06%)	⬇️
panel/io/state.py	69.18% <100.00%> (-0.11%)	⬇️
panel/io/reload.py	69.23% <0.00%> (-2.20%)	⬇️

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