auth: remove index out of range in role grant
auth: remove index out of range in role grant

Fixes https://github.com/coreos/etcd/issues/5077

jlamillan/etcd,jonboulle/etcd,sinsharat/etcd,ravigadde/etcd,vimalk78/etcd,rchicoli/etcd,sofuture/etcd,a-robinson/etcd,mqliang/etcd,sebrandon1/etcd,swingbach/etcd,armstrongli/etcd,heyitsanthony/etcd,ajityagaty/etcd,mickep76/etcd,rnaveiras/etcd,rchicoli/etcd,hongchaodeng/etcd,armstrongli/etcd,luxas/etcd,li-ang/etcd,lorneli/etcd,glevand/coreos--etcd,insequent/etcd,li-ang/etcd,mqliang/etcd,mkumatag/etcd,mkumatag/etcd,dnaeon/etcd,rnaveiras/etcd,mqliang/etcd,coreos/etcd,OyTao/etcd_learning,tguangch/etcd,coreos/etcd,sofuture/etcd,endocode/etcd,qqsun8819/etcd,plasticbox/etcd,bdarnell/etcd,heyitsanthony/etcd,bdarnell/etcd,swingbach/etcd,endocode/etcd,qqsun8819/etcd,chipironcin/etcd,sebrandon1/etcd,CrazyJvm/etcd,purpleidea/etcd,linuxcer/etcd,rnaveiras/etcd,OyTao/etcd_learning,joshix/etcd,cockroachdb/etcd,mkumatag/etcd,ktateish/etcd,plasticbox/etcd,xiang90/etcd,achanda/etcd,tguangch/etcd,visheshnp/etcd,westhood/etcd,glevand/coreos--etcd,etcd-io/etcd,achanda/etcd,CrazyJvm/etcd,zaphod-concur/etcd,hongchaodeng/etcd,nekto0n/etcd,wantonsolutions/dinv-etcd,mickep76/etcd,jonboulle/etcd,akauppi/etcd,ajityagaty/etcd,yudai/etcd,a-robinson/etcd,ajityagaty/etcd,glycerine/etcd,joshix/etcd,xiang90/etcd,westhood/etcd,luxas/etcd,lorneli/etcd,jlamillan/etcd,visheshnp/etcd,gyuho/etcd,ktateish/etcd,yudai/etcd,wantonsolutions/dinv-etcd,cockroachdb/etcd,YuleiXiao/etcd,lumjjb/etcd,YuleiXiao/etcd,bluebreezecf/etcd,dannysauer/etcd,StorReduce/etcd,chipironcin/etcd,insequent/etcd,glycerine/etcd,fanminshi/etcd,dannysauer/etcd,purpleidea/etcd,StorReduce/etcd,wantonsolutions/dinv-etcd,fanminshi/etcd,whuwxl/etcd,lumjjb/etcd,handsomesun/etcd,dnaeon/etcd,sinsharat/etcd,akauppi/etcd,gyuho/etcd,etcd-io/etcd,nekto0n/etcd,zaphod-concur/etcd,ravigadde/etcd,handsomesun/etcd,linuxcer/etcd,etcd-io/etcd,bluebreezecf/etcd,vimalk78/etcd,whuwxl/etcd