Ignore invalid mouse cursor image

webrtc/modules/desktop_capture/mouse_cursor_monitor_mac.mm

Index: webrtc/modules/desktop_capture/mouse_cursor_monitor_mac.mm
diff --git a/webrtc/modules/desktop_capture/mouse_cursor_monitor_mac.mm b/webrtc/modules/desktop_capture/mouse_cursor_monitor_mac.mm
index 2bb06b7b4bcb0ce599ed1f7c5ef194fbc94b9f1b..371852bf42b2fd3f01b7c14704d3698ed8baec7e 100644
--- a/webrtc/modules/desktop_capture/mouse_cursor_monitor_mac.mm
+++ b/webrtc/modules/desktop_capture/mouse_cursor_monitor_mac.mm
@@ -256,6 +256,9 @@ void MouseCursorMonitorMac::CaptureImage(float scale) {
   NSCursor* nscursor = [NSCursor currentSystemCursor];
 
   NSImage* nsimage = [nscursor image];
+  if (nsimage == nil || !nsimage.isValid) {
+    return;
+  }
   NSSize nssize = [nsimage size];  // DIP size
 
   // No need to caputre cursor image if it's unchanged since last capture.