etcdserver: fix panic when getting header of raft request
etcdserver: fix panic when getting header of raft request

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