fix a couple of bugs from the recent update.
fix a couple of bugs from the recent update.

R=rsc
http://codereview.appspot.com/1710042

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