http2: correct ServeConnOpts.context's nil receiver check

Fixes golang/go#33839

Change-Id: Ic1a9e42afc8efda7ec2d39e705efe41474237d82
Reviewed-on: https://go-review.googlesource.com/c/net/+/191857
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

http2/server.go

@@ -322,7 +322,7 @@ type ServeConnOpts struct {
 }
 
 func (o *ServeConnOpts) context() context.Context {
-	if o.Context != nil {
+	if o != nil && o.Context != nil {
 		return o.Context
 	}
 	return context.Background()

http2/server_test.go

@@ -3220,6 +3220,26 @@ func (c *issue53Conn) SetDeadline(t time.Time) error      { return nil }
 func (c *issue53Conn) SetReadDeadline(t time.Time) error  { return nil }
 func (c *issue53Conn) SetWriteDeadline(t time.Time) error { return nil }
 
+// golang.org/issue/33839
+func TestServeConnOptsNilReceiverBehavior(t *testing.T) {
+	defer func() {
+		if r := recover(); r != nil {
+			t.Errorf("got a panic that should not happen: %v", r)
+		}
+	}()
+
+	var o *ServeConnOpts
+	if o.context() == nil {
+		t.Error("o.context should not return nil")
+	}
+	if o.baseConfig() == nil {
+		t.Error("o.baseConfig should not return nil")
+	}
+	if o.handler() == nil {
+		t.Error("o.handler should not return nil")
+	}
+}
+
 // golang.org/issue/12895
 func TestConfigureServer(t *testing.T) {
 	tests := []struct {