gettext: Integer overflow leads to heap OOB write or read

gettext: Integer overflow leads to heap OOB write or read

Calculation of ctx->grub_gettext_msg_list size in grub_mofile_open() may
overflow leading to subsequent OOB write or read. This patch fixes the
issue by replacing grub_zalloc() and explicit multiplication with
grub_calloc() which does the same thing in safe manner.

Fixes: CVE-2024-45776
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>