Fix nil pointer in screenshot (#699)

Fix Panic in Screenshot #631

page.go

@@ -400,6 +400,10 @@ func (p *Page) Screenshot(fullpage bool, req *proto.PageCaptureScreenshot) ([]by
 			return nil, err
 		}
 
+		if metrics.CSSContentSize == nil {
+			return nil, errors.New("failed to get css content size")
+		}
+
 		oldView := proto.EmulationSetDeviceMetricsOverride{}
 		set := p.LoadState(&oldView)
 		view := oldView

page_test.go

@@ -19,6 +19,7 @@ import (
 	"github.com/go-rod/rod/lib/devices"
 	"github.com/go-rod/rod/lib/proto"
 	"github.com/go-rod/rod/lib/utils"
+	"github.com/ysmood/gson"
 )
 
 func TestGetPageBrowser(t *testing.T) {
@@ -657,6 +658,13 @@ func TestScreenshotFullPage(t *testing.T) {
 		g.mc.stubErr(1, proto.EmulationSetDeviceMetricsOverride{})
 		p.MustScreenshotFullPage()
 	})
+
+	g.Panic(func() {
+		g.mc.stub(1, proto.PageGetLayoutMetrics{}, func(send StubSend) (gson.JSON, error) {
+			return gson.New(proto.PageGetLayoutMetricsResult{}), nil
+		})
+		p.MustScreenshotFullPage()
+	})
 }
 
 func TestScreenshotFullPageInit(t *testing.T) {