Bug 1743095 [Linux] Check mGdkWindow state at is_mouse_in_window() r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D132237

widget/gtk/nsWindow.cpp

@@ -7156,6 +7156,11 @@ MOZ_CAN_RUN_SCRIPT static void WaylandDragWorkaround(GdkEventButton* aEvent) {
 
 static bool is_mouse_in_window(GdkWindow* aWindow, gdouble aMouseX,
                                gdouble aMouseY) {
+  GdkWindow* window = aWindow;
+  if (!window) {
+    return false;
+  }
+
   gint x = 0;
   gint y = 0;
   gint w, h;
@@ -7163,8 +7168,6 @@ static bool is_mouse_in_window(GdkWindow* aWindow, gdouble aMouseX,
   gint offsetX = 0;
   gint offsetY = 0;
 
-  GdkWindow* window = aWindow;
-
   while (window) {
     gint tmpX = 0;
     gint tmpY = 0;