menu: fix unexpected behavior when a menu is opened from another menu

menu: fix unexpected behavior when a menu is opened from another menu

server->menu_current should be cleared before calling actions_run() as
it may internally call menu_open_root(). Clearing it after actions_run()
leads to an inconsistent state where a menu is opened but
server->menu_current is NULL. It even lead to a segfault when the item
opening another menu is contained in a pipemenu, because
menu_open_root() calls destroy_pipemenu() when server->menu_current is
set, which makes accessing item->actions a UAF.