Adds safety check before accessing "rule parent"

[kettanaito]

Adds an explicit safety check when accessing properties of rule.parent instance.

This fixes the issue I'm currently experiencing with the latest versions of all css-related dependencies installed. See #7 for more details from my side. Thanks.

GitHub

• Fixes Help with css-loader error #7

[codecov-io]

Codecov Report

Merging #15 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
+ Coverage   99.31%   99.31%   +<.01%     
==========================================
  Files           2        2              
  Lines         291      292       +1     
  Branches       91       92       +1     
==========================================
+ Hits          289      290       +1     
  Misses          2        2

Impacted Files	Coverage Δ
index.js	99.23% <100%> (ø)	⬆️

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[kettanaito]

Thank you, @evilebottnawi!