nconfig: Fix segfault when menu is empty

nconf crush with segfault if press right arrow in empty menu.

Signed-off-by: Andrej Gelenberg <andrej.gelenberg@udo.edu>
Signed-off-by: Michal Marek <mmarek@suse.cz>
diff --git a/scripts/kconfig/nconf.c b/scripts/kconfig/nconf.c
index 762caf8..2ba71bc 100644
--- a/scripts/kconfig/nconf.c
+++ b/scripts/kconfig/nconf.c
@@ -676,6 +676,8 @@
 	struct mitem *mcur;
 
 	cur = current_item(curses_menu);
+	if (!cur)
+		return NULL;
 	mcur = (struct mitem *) item_userptr(cur);
 	return mcur->usrptr;