cocoa: Don't crash on messagebox keypress without a successful SDL_Init().

src/video/cocoa/SDL_cocoakeyboard.m

@@ -646,7 +646,7 @@ Cocoa_SetTextInputRect(_THIS, SDL_Rect *rect)
 void
 Cocoa_HandleKeyEvent(_THIS, NSEvent *event)
 {
-    SDL_VideoData *data = (SDL_VideoData *) _this->driverdata;
+    SDL_VideoData *data = _this ? ((SDL_VideoData *) _this->driverdata) : NULL;
     if (!data) {
         return;  /* can happen when returning from fullscreen Space on shutdown */
     }