command: special case for "add /"

The undocumented command "add /" adds the full music database to the
playlist.  Don't interpret this special path as a local file path.

src/command.c

@@ -448,7 +448,7 @@ static int handleAdd(struct client *client,
 	char *path = argv[1];
 	enum playlist_result result;
 
-	if (path[0] == '/') {
+	if (path[0] == '/' && path[1] != 0) {
 		result = playlist_append_file(path, client_get_uid(client),
 					      NULL);
 		return print_playlist_result(client, result);