Initialize last_cursor_

This ensures that last_cursor_ is intialized before it is checked.
This code is in the Firefox repo with the original review here:
https://phabricator.services.mozilla.com/D113833

Bug: webrtc:13481
Change-Id: I806009cba1aba193ab12bb86e39f98c56043000f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/239725
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Stefan Holmer <stefan@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35509}
diff --git a/modules/desktop_capture/mouse_cursor_monitor_mac.mm b/modules/desktop_capture/mouse_cursor_monitor_mac.mm
index 995aeb9..3db4332c 100644
--- a/modules/desktop_capture/mouse_cursor_monitor_mac.mm
+++ b/modules/desktop_capture/mouse_cursor_monitor_mac.mm
@@ -75,9 +75,9 @@
   rtc::scoped_refptr<DesktopConfigurationMonitor> configuration_monitor_;
   CGWindowID window_id_;
   ScreenId screen_id_;
-  Callback* callback_;
+  Callback* callback_ = NULL;
   Mode mode_;
-  __strong NSImage* last_cursor_;
+  __strong NSImage* last_cursor_ = NULL;
 };
 
 MouseCursorMonitorMac::MouseCursorMonitorMac(const DesktopCaptureOptions& options,
@@ -86,7 +86,6 @@
     : configuration_monitor_(options.configuration_monitor()),
       window_id_(window_id),
       screen_id_(screen_id),
-      callback_(NULL),
       mode_(SHAPE_AND_POSITION) {
   RTC_DCHECK(window_id == kCGNullWindowID || screen_id == kInvalidScreenId);
 }