Fix an error when io.CopyBuffer returns n=0

It could be that the "n=0 means EOF" semantics are leaking from the
underlying splice() system call? Anyway this prevents radiod from
getting stuck in a busy loop in certain(?) cases.

node/proxy.go

@@ -175,6 +175,10 @@ func copyStream(tag string, out, in *net.TCPConn, promCounter prometheus.Counter
 			}
 			return
 		}
+		if n == 0 {
+			log.Printf("http: proxy got 0 bytes from splice()")
+			return
+		}
 	}
 }