Fix ifcIndex usage in Windows

[edaniels]

We were not doing the ifcIndex check correctly which is a bug everywhere but only shows on Windows right now due to not seeing Control Messages.

Fixes #118

[codecov]

Codecov Report

Attention: Patch coverage is 41.66667% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 65.75%. Comparing base (1f6facc) to head (a4cb377).

Files	Patch %	Lines
conn.go	41.66%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #184      +/-   ##
==========================================
- Coverage   66.54%   65.75%   -0.80%     
==========================================
  Files           1        1              
  Lines         837      838       +1     
==========================================
- Hits          557      551       -6     
- Misses        204      209       +5     
- Partials       76       78       +2     

Flag	Coverage Δ
go	65.75% <41.66%> (-0.80%)	⬇️
wasm	0.00% <0.00%> (ø)

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