cocoa: Don't crash if moving a message box created before SDL_Init.

Fixes #5928.

src/video/cocoa/SDL_cocoamouse.m

@@ -400,6 +400,12 @@ Cocoa_HandleTitleButtonEvent(_THIS, NSEvent *event)
     SDL_Window *window;
     NSWindow *nswindow = [event window];
 
+    /* You might land in this function before SDL_Init if showing a message box.
+       Don't derefence a NULL pointer if that happens. */
+    if (_this == NULL) {
+        return;
+    }
+
     for (window = _this->windows; window; window = window->next) {
         SDL_WindowData *data = (__bridge SDL_WindowData *)window->driverdata;
         if (data && data.nswindow == nswindow) {