Fixed: prevent node exception if parent does not exist or is undefined
Fixed: prevent node exception if parent does not exist or is undefined

Postcss api should not bring an undefined parent for an attached node. However some manual manipulations might create bring this state and boom, pixrem blow up because a low level test have not been done before.
You should always double check that an object exist before calling a prop on it, since `undefined` me be there... Thats why I do not provide any test. Tests will be added to the responsible for this state. 

Ref https://github.com/cssnext/cssnext/issues/67
robwierzbowski/node-pixrem,cbas/node-pixrem