Fix error handling in X11 screen capturer

webrtc/modules/desktop_capture/screen_capturer_x11.cc

Index: webrtc/modules/desktop_capture/screen_capturer_x11.cc
diff --git a/webrtc/modules/desktop_capture/screen_capturer_x11.cc b/webrtc/modules/desktop_capture/screen_capturer_x11.cc
index b4f3bf575b84e64acb4f3d231c0d024bff50c365..ed31774070805716826f913ad7dd33458c5f560c 100644
--- a/webrtc/modules/desktop_capture/screen_capturer_x11.cc
+++ b/webrtc/modules/desktop_capture/screen_capturer_x11.cc
@@ -340,7 +340,8 @@ std::unique_ptr<DesktopFrame> ScreenCapturerLinux::CaptureScreen() {
     // Doing full-screen polling, or this is the first capture after a
     // screen-resolution change.  In either case, need a full-screen capture.
     DesktopRect screen_rect = DesktopRect::MakeSize(frame->size());
-    x_server_pixel_buffer_.CaptureRect(screen_rect, frame.get());
+    if (!x_server_pixel_buffer_.CaptureRect(screen_rect, frame.get()))
+      return nullptr;
     updated_region->SetRect(screen_rect);
   }