Fix DesktopCapturer constructors to handle failures properly.

webrtc/modules/desktop_capture/desktop_capturer.cc

Index: webrtc/modules/desktop_capture/desktop_capturer.cc
diff --git a/webrtc/modules/desktop_capture/desktop_capturer.cc b/webrtc/modules/desktop_capture/desktop_capturer.cc
index 37b3396df9ea45e4a78047fc67aaeb8a5e997be4..e655474e70a18fa79b36cb7cc6fdac12380bcc9f 100644
--- a/webrtc/modules/desktop_capture/desktop_capturer.cc
+++ b/webrtc/modules/desktop_capture/desktop_capturer.cc
@@ -38,7 +38,7 @@ bool DesktopCapturer::FocusOnSelectedSource() {
 std::unique_ptr<DesktopCapturer> DesktopCapturer::CreateWindowCapturer(
     const DesktopCaptureOptions& options) {
   std::unique_ptr<DesktopCapturer> capturer = CreateRawWindowCapturer(options);
-  if (options.detect_updated_region()) {
+  if (capturer && options.detect_updated_region()) {
     capturer.reset(new DesktopCapturerDifferWrapper(std::move(capturer)));
   }
 
@@ -49,7 +49,7 @@ std::unique_ptr<DesktopCapturer> DesktopCapturer::CreateWindowCapturer(
 std::unique_ptr<DesktopCapturer> DesktopCapturer::CreateScreenCapturer(
     const DesktopCaptureOptions& options) {
   std::unique_ptr<DesktopCapturer> capturer = CreateRawScreenCapturer(options);
-  if (options.detect_updated_region()) {
+  if (capturer && options.detect_updated_region()) {
     capturer.reset(new DesktopCapturerDifferWrapper(std::move(capturer)));
   }