fix: potential race when reading incomingErr

[rvagg]

Reported by @parthshah1, discovered via Antithesis. A client is able to get itself into a state where there's a race in accessing incomingErr, so we'll lock it just to be safe.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.

Please upload report for BASE (master@9a16432). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
websocket.go	75.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #125   +/-   ##
=========================================
  Coverage          ?   71.49%           
=========================================
  Files             ?       12           
  Lines             ?     1968           
  Branches          ?        0           
=========================================
  Hits              ?     1407           
  Misses            ?      460           
  Partials          ?      101           

Files with missing lines	Coverage Δ
websocket.go	75.30% <75.00%> (ø)

[Stebalien]

Feels like there should be a better way but this is definitely the safest approach.