SDL_UDEV_DelCallback: return early if _this is NULL

Fixes https://github.com/libsdl-org/SDL/issues/6548

src/core/linux/SDL_udev.c

@@ -545,6 +545,10 @@ SDL_UDEV_DelCallback(SDL_UDEV_Callback cb)
     SDL_UDEV_CallbackList *item;
     SDL_UDEV_CallbackList *prev = NULL;
 
+    if (_this == NULL) {
+        return;
+    }
+
     for (item = _this->first; item != NULL; item = item->next) {
         /* found it, remove it. */
         if (item->callback == cb) {