fix: fix sftp key loading

Signed-off-by: Robin Appelman <robin@icewind.nl>

Author: icewind1991

apps/files_external/lib/Lib/Auth/PublicKey/RSAPrivateKey.php

@@ -64,6 +64,10 @@ public function manipulateStorageConfig(StorageConfig &$storage, IUser $user = n
 				throw new \RuntimeException('unable to load private key');
 			}
 		}
+
+		if (!$auth instanceof RSA\PrivateKey) {
+			throw new \RuntimeException('Loaded key is not a private key');
+		}
 		$storage->setBackendOption('public_key_auth', $auth);
 	}
 }