Validate window pointer before sending events for it

If there are bugs it's possible that the window pointer is invalid. Double check it before sending the application events and potentially dereferencing it.

This showed up in libsdl-org#10494 as a window getting mouse focus as it was being destroyed and then crashing later when mouse focus was set to a different window.

src/events/SDL_windowevents.c

@@ -43,7 +43,7 @@ int SDL_SendWindowEvent(SDL_Window *window, SDL_EventType windowevent,
 {
     int posted;
 
-    if (!window) {
+    if (!SDL_ObjectValid(window, SDL_OBJECT_TYPE_WINDOW)) {
         return 0;
     }
     if (window->is_destroying && windowevent != SDL_EVENT_WINDOW_DESTROYED) {