CLEANUP: Free old_argv on deinit

This allocation technically is always reachable and cannot leak, however other
global variables such as `oldpids` are already being freed. This is in an
attempt to get HAProxy to a state where there are zero live allocations after a
clean exit.

src/haproxy.c

@@ -2680,6 +2680,7 @@ void deinit(void)
 	free(global.node);    global.node = NULL;
 	free(global.desc);    global.desc = NULL;
 	free(oldpids);        oldpids = NULL;
+	free(old_argv);       old_argv = NULL;
 	free(localpeer);      localpeer = NULL;
 	task_destroy(idle_conn_task);
 	idle_conn_task = NULL;