script/execute: Don't crash on a "for" loop with no items

The following crashes the parser:

  for x in; do
  0
  done

This is because grub_script_arglist_to_argv() doesn't consider the
possibility that arglist is NULL. Catch that explicitly.

This avoids a NULL pointer dereference.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/script/execute.c

@@ -624,6 +624,9 @@ grub_script_arglist_to_argv (struct grub_script_arglist *arglist,
   struct grub_script_arg *arg = 0;
   struct grub_script_argv result = { 0, 0, 0 };
 
+  if (arglist == NULL)
+    return 1;
+
   for (; arglist && arglist->arg; arglist = arglist->next)
     {
       if (grub_script_argv_next (&result))