server: prevent race during resched pump on a closed stream
server: prevent race during resched pump on a closed stream

Fixes #44

golang/net,cesanta/goxnet,cmars/tools,DramaFever/golang.org-x-net,DramaFever/golang.org-x-net,gomini/net,cmars/tools,cmars/tools,cmars/tools,raboof/net,cesanta/goxnet,gomini/net,raboof/net,albertjin/golang-net,albertjin/golang-net