pool: fix panic caused by sending to closed channel, fix #237
pool: fix panic caused by sending to closed channel, fix #237

Hurricanezwf/frp,KunTengRom/xfrps,KunTengRom/xfrps,bingtianbaihua/frp,KunTengRom/xfrps,Hurricanezwf/frp,fatedier/frp,bingtianbaihua/frp,bingtianbaihua/frp,fatedier/frp,Hurricanezwf/frp,fatedier/frp,Hurricanezwf/frp,fatedier/frp,KunTengRom/xfrps,bingtianbaihua/frp