Fix data race on listener.connWG

[stv0g]

Fixes #259

[codecov[bot]]

Codecov Report

Patch coverage: 85.18% and project coverage change: -0.11% :warning:

Comparison is base (6890c79) 82.21% compared to head (e209826) 82.11%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #260      +/-   ##
==========================================
- Coverage   82.21%   82.11%   -0.11%     
==========================================
  Files          37       38       +1     
  Lines        3217     3243      +26     
==========================================
+ Hits         2645     2663      +18     
- Misses        456      461       +5     
- Partials      116      119       +3     

Flag	Coverage Δ
go	81.86% <85.18%> (-0.20%)	:arrow_down:
wasm	74.59% <ø> (ø)

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Files Changed	Coverage Δ
internal/sync/waitgroup.go	84.61% <84.61%> (ø)
udp/conn.go	83.03% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[edaniels]

It looks like sync.WaitGroup is in use in transport for connWG and there's no current issues around it being used, so I'll close this issue for now.