manager: Cleanup manager_channelvars during shutdown.

ASTERISK-25680 #close

Change-Id: I3251d781cbc3f48a6a7e1b969ac4983f552b2446

main/manager.c

@@ -7600,6 +7600,8 @@ static void manager_shutdown(void)
 	while ((user = AST_LIST_REMOVE_HEAD(&users, list))) {
 		manager_free_user(user);
 	}
+
+	ast_free(manager_channelvars);
 }
 
 static void manager_set_defaults(void)