Ignore invalid mouse cursor image

A crash has been randomly detected across different versions. The NSImage
crashes the binary in its lockFocusFlipped() function. The suspicious issue is
that NSCursor::image() returns an invalid NSImage.

BUG=chromium:752036

Review-Url: https://codereview.webrtc.org/2993173003
Cr-Commit-Position: refs/heads/master@{#19273}

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.