Safety fix: clear "_this->wakeup_window" when destroying the window

(cherry picked from commit c838ccf0e3a330ebbcf433698b8f5370c6fa80d6)

src/video/SDL_video.c

@@ -3276,6 +3276,10 @@ void SDL_DestroyWindow(SDL_Window *window)
         _this->current_glwin = NULL;
     }
 
+    if (_this->wakeup_window == window) {
+        _this->wakeup_window = NULL;
+    }
+
     /* Now invalidate magic */
     window->magic = NULL;