Fix an @extend edge case with child selectors.
Fix an @extend edge case with child selectors.

Closes #1789

HugoRLopes/sass,nickg33/sass,stewx/sass,ridixcr/sass,liquidmetal/sass,srawlins/sass,StephanieMak/sass,DanteKaris/sass,OrenBochman/sass,MetaAshley/sass,modulexcite/sass,Anshdesire/sass,rammy7219/sass,Galactix/sass,nicklima/sass,nicklima/sass,srawlins/sass,OrenBochman/sass,Industrys/sass,lmetla/sass,Andrey-Pavlov/sass,saper/sass,amitsuroliya/sass,DanteKaris/sass,hcatlin/sass,amitsuroliya/sass,stewx/sass,Anshdesire/sass,antonifs/sass,ridixcr/sass,nickg33/sass,askl56/sass,MetaAshley/sass,cmpereirasi/sass,StephanieMak/sass,cmpereirasi/sass,jaroot32/sass,saper/sass,modulexcite/sass,jaroot32/sass,zaru/sass,lmetla/sass,zaru/sass,Galactix/sass,schneems/sass,schneems/sass,rammy7219/sass,askl56/sass,Industrys/sass,Andrey-Pavlov/sass,antonifs/sass,hcatlin/sass,xzyfer/sass,liquidmetal/sass,HugoRLopes/sass,xzyfer/sass