commands/extcmd: Missing check for failed allocation

commands/extcmd: Missing check for failed allocation

The grub_extcmd_dispatcher() calls grub_arg_list_alloc() to allocate
a grub_arg_list struct but it does not verify the allocation was successful.
In case of failed allocation the NULL state pointer can be accessed in
parse_option() through grub_arg_parse() which may lead to a security issue.

Fixes: CVE-2024-45775
Reported-by: Nils Langius <nils@langius.de>
Signed-off-by: Lidong Chen <lidong.chen@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Alec Brown <alec.r.brown@oracle.com>