fix(external): Get storage before update it

Signed-off-by: Joas Schilling <coding@schilljs.com>

apps/files_external/lib/Service/UserStoragesService.php

@@ -124,6 +124,9 @@ public function addStorage(StorageConfig $newStorage) {
 	 * @throws NotFoundException if the given storage does not exist in the config
 	 */
 	public function updateStorage(StorageConfig $updatedStorage) {
+		// verify ownership through $this->isApplicable() and otherwise throws an exception
+		$this->getStorage($updatedStorage->getId());
+
 		$updatedStorage->setApplicableUsers([$this->getUser()->getUID()]);
 		return parent::updateStorage($updatedStorage);
 	}