goprotobuf: more cleanup suggested by rsc.
goprotobuf: more cleanup suggested by rsc.

R=rsc
CC=golang-dev
http://codereview.appspot.com/4388052

adwinsky/goprotobuf,supiyun/goprotobuf,t3rm1n4l/goprotobuf,polydawn-ports/goprotobuf,liumuqi/goprotobuf,tonnerre/go-protobuf,ruiaylin/goprotobuf,wfxiang08/goprotobuf