consensus: fix M_Cmd.Format crash on a null pointer
consensus: fix M_Cmd.Format crash on a null pointer

bitly/doozerd,ha/doozerd,magastzheng/doozerd,ha/doozerd,soundcloud/doozerd,coreos/doozerd,rowhit/doozerd,npchp110/doozerd,jmptrader/doozerd,coreos/doozer,tempbottle/doozerd,jmptrader/doozerd,soundcloud/doozerd,soundcloud/doozer,ylong/doozerd,ha/doozerd,npchp110/doozerd,tempbottle/doozerd,zyxar/doozerd,rowhit/doozerd,zyxar/doozerd,soundcloud/doozerd,ylong/doozerd,magastzheng/doozerd,tempbottle/doozerd,npchp110/doozer,bitly/doozerd,bitly/doozerd,ylong/doozerd,rowhit/doozerd,jmptrader/doozer,soundcloud/doozer,npchp110/doozerd,soundcloud/doozerd,tempbottle/doozerd,coreos/doozerd,zyxar/doozerd,npchp110/doozerd,jmptrader/doozerd,zyxar/doozerd,jmptrader/doozerd,magastzheng/doozerd,rowhit/doozerd,coreos/doozerd,magastzheng/doozerd,ylong/doozerd,ha/doozer