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

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>