Avoid calling 'get_next_substitution_result' when at end of string
Avoid calling 'get_next_substitution_result' when at end of string

This fixes failing tests in S05-transliteration/with-closure.

ab5tract/rakudo,labster/rakudo,ugexe/rakudo,ugexe/rakudo,Leont/rakudo,nunorc/rakudo,zostay/rakudo,cygx/rakudo,ungrim97/rakudo,awwaiid/rakudo,softmoth/rakudo,zhuomingliang/rakudo,usev6/rakudo,tbrowder/rakudo,nbrown/rakudo,usev6/rakudo,usev6/rakudo,awwaiid/rakudo,tony-o/rakudo,samcv/rakudo,ab5tract/rakudo,samcv/rakudo,nunorc/rakudo,nbrown/rakudo,tbrowder/rakudo,niner/rakudo,raydiak/rakudo,b2gills/rakudo,awwaiid/rakudo,cognominal/rakudo,azawawi/rakudo,skids/rakudo,paultcochrane/rakudo,niner/rakudo,jonathanstowe/rakudo,labster/rakudo,labster/rakudo,rakudo/rakudo,sergot/rakudo,sjn/rakudo,samcv/rakudo,paultcochrane/rakudo,azawawi/rakudo,lucasbuchala/rakudo,cognominal/rakudo,Leont/rakudo,raydiak/rakudo,paultcochrane/rakudo,awwaiid/rakudo,cognominal/rakudo,cognominal/rakudo,b2gills/rakudo,nunorc/rakudo,niner/rakudo,LLFourn/rakudo,salortiz/rakudo,awwaiid/rakudo,Leont/rakudo,ab5tract/rakudo,b2gills/rakudo,MasterDuke17/rakudo,LLFourn/rakudo,tbrowder/rakudo,labster/rakudo,tony-o/rakudo,tony-o/rakudo,sergot/rakudo,azawawi/rakudo,jonathanstowe/rakudo,Gnouc/rakudo,MasterDuke17/rakudo,LLFourn/rakudo,rakudo/rakudo,salortiz/rakudo,LLFourn/rakudo,lucasbuchala/rakudo,rakudo/rakudo,salortiz/rakudo,lucasbuchala/rakudo,sjn/rakudo,ugexe/rakudo,Gnouc/rakudo,skids/rakudo,salortiz/rakudo,laben/rakudo,LLFourn/rakudo,jonathanstowe/rakudo,sergot/rakudo,tony-o/rakudo,zhuomingliang/rakudo,dankogai/rakudo,tbrowder/rakudo,MasterDuke17/rakudo,labster/rakudo,zostay/rakudo,paultcochrane/rakudo,ugexe/rakudo,nbrown/rakudo,usev6/rakudo,tony-o/rakudo,dankogai/rakudo,cygx/rakudo,softmoth/rakudo,labster/rakudo,skids/rakudo,jonathanstowe/rakudo,nbrown/rakudo,azawawi/rakudo,lucasbuchala/rakudo,laben/rakudo,usev6/rakudo,dankogai/rakudo,dankogai/rakudo,skids/rakudo,ungrim97/rakudo,sergot/rakudo,softmoth/rakudo,zostay/rakudo,Gnouc/rakudo,ungrim97/rakudo,ungrim97/rakudo,MasterDuke17/rakudo,rakudo/rakudo,laben/rakudo,tbrowder/rakudo,rakudo/rakudo,cygx/rakudo,Leont/rakudo,b2gills/rakudo,laben/rakudo,ab5tract/rakudo,cognominal/rakudo,MasterDuke17/rakudo,sjn/rakudo,skids/rakudo,dankogai/rakudo,cygx/rakudo,raydiak/rakudo,paultcochrane/rakudo,Gnouc/rakudo,tony-o/rakudo,rakudo/rakudo,ab5tract/rakudo,softmoth/rakudo,sjn/rakudo,zhuomingliang/rakudo,ugexe/rakudo,sjn/rakudo,salortiz/rakudo,softmoth/rakudo,salortiz/rakudo,b2gills/rakudo,nbrown/rakudo,nbrown/rakudo,MasterDuke17/rakudo,tbrowder/rakudo,zostay/rakudo,Gnouc/rakudo,nunorc/rakudo,lucasbuchala/rakudo,azawawi/rakudo,samcv/rakudo,niner/rakudo,zhuomingliang/rakudo,raydiak/rakudo,jonathanstowe/rakudo,samcv/rakudo,Gnouc/rakudo,cygx/rakudo,ungrim97/rakudo