net/http/httptest: don't panic on Close of user-constructed Server value

If the user created an httptest.Server directly without using a
constructor it won't have the new unexported 'client' field. So don't
assume it's non-nil.

Fixes #19729

Change-Id: Ie92e5da66cf4e7fb8d95f3ad0f4e3987d3ae8b77
Reviewed-on: https://go-review.googlesource.com/38710
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Kevin Burke <kev@inburke.com>

src/net/http/httptest/server.go

@@ -209,8 +209,10 @@ func (s *Server) Close() {
 	}
 
 	// Also close the client idle connections.
-	if t, ok := s.client.Transport.(closeIdleTransport); ok {
-		t.CloseIdleConnections()
+	if s.client != nil {
+		if t, ok := s.client.Transport.(closeIdleTransport); ok {
+			t.CloseIdleConnections()
+		}
 	}
 
 	s.wg.Wait()

src/net/http/httptest/server_test.go

@@ -145,3 +145,20 @@ func TestTLSServerClientTransportType(t *testing.T) {
 		t.Errorf("got %T, want *http.Transport", client.Transport)
 	}
 }
+
+type onlyCloseListener struct {
+	net.Listener
+}
+
+func (onlyCloseListener) Close() error { return nil }
+
+// Issue 19729: panic in Server.Close for values created directly
+// without a constructor (so the unexported client field is nil).
+func TestServerZeroValueClose(t *testing.T) {
+	ts := &Server{
+		Listener: onlyCloseListener{},
+		Config:   &http.Server{},
+	}
+
+	ts.Close() // tests that it doesn't panic
+}