Assert a valid window pointer instead of dismissing an invalid pointer

Throwing an assertion and breaking the program should allow developers
to catch wrong usage of the API faster.

Follow-up of 9493e6974f70a906950c9d8196c9a9982300f261

src/events/SDL_windowevents.c

@@ -43,9 +43,11 @@ int SDL_SendWindowEvent(SDL_Window *window, SDL_EventType windowevent,
 {
     int posted;
 
-    if (!SDL_ObjectValid(window, SDL_OBJECT_TYPE_WINDOW)) {
+    if (!window) {
         return 0;
     }
+    SDL_assert(SDL_ObjectValid(window, SDL_OBJECT_TYPE_WINDOW));
+
     if (window->is_destroying && windowevent != SDL_EVENT_WINDOW_DESTROYED) {
         return 0;
     }