acpi(4): Fix out of range access in System States

Hi,

When booting a snapshot kernel on the "cloud hypervisor" [1] based on
Linux/KVM, it runs straight into a protection fault in acpi(4):

OpenBSD 7.9-current (GENERIC.MP) #24: Wed Jul  8 04:05:05 MDT 2026
    [email protected]:/usr/src/sys/arch/amd64/compile/GENERIC.MP
real mem = 4262391808 (4064MB)
avail mem = 4111220736 (3920MB)
random: good seed from bootblocks
mpath0 at root
scsibus0 at mpath0: 256 targets
mainbus0 at root
bios0 at mainbus0: SMBIOS rev. 2.8 @ 0xbf13e000 (3 entries)
bios0: vendor cloud-hypervisor version "0"
efi0 at bios0: UEFI 2.7
efi0: EDK II rev 0x10000
acpi0 at bios0: ACPI 6.3
acpi0: sleep stateskernel: protection fault trap, code=0
Stopped at      aml_val2int+0x25:       movl    0(%rdi),%eax
ddb{0}>

This is caused by blindly reading the "System State Package" without
checking the length of the package in before.  In the ACPI spec it looks like
there have to be 4 values.  But, the second one has the comment:

	On HW-reduced platforms, this value is ignored.

The Linux kernel also does a length check before touching the package elements.
So, I would just add the following length here check, too.

ok?

bye,
jan

[1]: https://www.cloudhypervisor.org/
[2]: https://uefi.org/specs/ACPI/6.5/07_Power_and_Performance_Mgmt.html#sx-system-states

Index: dev/acpi/acpi.c
===================================================================
RCS file: /cvs/src/sys/dev/acpi/acpi.c,v
diff -u -p -r1.457 acpi.c
--- dev/acpi/acpi.c	11 Mar 2026 16:18:42 -0000	1.457
+++ dev/acpi/acpi.c	8 Jul 2026 21:07:03 -0000
@@ -2569,7 +2569,7 @@ acpi_init_states(struct acpi_softc *sc)
 		sc->sc_sleeptype[i].slp_typb = -1;
 		if (aml_evalname(sc, sc->sc_root, name, 0, NULL, &res) != 0)
 			continue;
-		if (res.type != AML_OBJTYPE_PACKAGE) {
+		if (res.type != AML_OBJTYPE_PACKAGE || res.length < 2) {
 			aml_freevalue(&res);
 			continue;
 		}