Preallocate to please linter

[ldemailly]

Avoids (new in 2.8.0)

$ golangci-lint run
dnsping.go:81:3: Consider preallocating keys (prealloc)
                keys := []string{}
                ^
1 issues:
* prealloc: 1

will fix #156 build once merged and rebased

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.32%. Comparing base (29fb97a) to head (b157a87).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #157   +/-   ##
=======================================
  Coverage   91.32%   91.32%           
=======================================
  Files           1        1           
  Lines         173      173           
=======================================
  Hits          158      158           
  Misses         11       11           
  Partials        4        4           

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