proxy: Fix a potential panic handling error states.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)

daemon/networkdriver/portmapper/proxy.go

@@ -130,7 +130,12 @@ func (p *proxyCommand) Start() error {
 		r.Read(buf)
 
 		if string(buf) != "0\n" {
-			errStr, _ := ioutil.ReadAll(r)
+			errStr, err := ioutil.ReadAll(r)
+			if err != nil {
+				errchan <- fmt.Errorf("Error reading exit status from userland proxy: %v", err)
+				return
+			}
+
 			errchan <- fmt.Errorf("Error starting userland proxy: %s", errStr)
 			return
 		}