fix: panic when write on closed channel

[jmattheis]

This fixes panic: send on closed channel which can be triggered by "POSTing /message" and then closing the websocket connection client side.

Reported privately by @skeletonsec.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.47%. Comparing base (6933034) to head (eec23c7).

Files with missing lines	Patch %	Lines
api/stream/stream.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1035      +/-   ##
==========================================
- Coverage   75.47%   75.47%   -0.01%     
==========================================
  Files          66       66              
  Lines        3556     3559       +3     
==========================================
+ Hits         2684     2686       +2     
- Misses        662      663       +1     
  Partials      210      210              

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