fix a panic when netns is called with 0 arguments

[nicks]

No description provided.

[codecov-io]

Codecov Report

Merging #6 into master will decrease coverage by 11.58%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #6       +/-   ##
==========================================
- Coverage   48.88%   37.3%   -11.59%     
==========================================
  Files           5       7        +2     
  Lines         270     386      +116     
==========================================
+ Hits          132     144       +12     
- Misses         96     194       +98     
- Partials       42      48        +6

Impacted Files	Coverage Δ
main.go	13.63% <0%> (ø)
netutils/netutils.go	0% <0%> (ø)

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[jessfraz]

Thanks!

[nicks]

thank you as well!