expvar: fix build

Change-Id: I6ee7aa76673e51576b5a84c512b22a0f8af5b02f
Reviewed-on: https://go-review.googlesource.com/3966
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/expvar/expvar_test.go

@@ -271,7 +271,7 @@ func BenchmarkRealworldExpvarUsage(b *testing.B) {
 	// Setup P client/server connections.
 	clients := make([]net.Conn, P)
 	servers := make([]net.Conn, P)
-	ln, err := net.Listen("tcp", laddr)
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
 	if err != nil {
 		b.Fatalf("Listen failed: %v", err)
 	}