ACPICA: Avoid walking the Namespace if start_node is NULL

Although commit 0c9992315e ("ACPICA: Avoid walking the ACPI Namespace
if it is not there") fixed the situation when both start_node and
acpi_gbl_root_node are NULL, the Linux kernel mainline now still crashed
on Honor Magicbook 14 Pro [1].

That happens due to the access to the member of parent_node in
acpi_ns_get_next_node().  The NULL pointer dereference will always
happen, no matter whether or not the start_node is equal to
ACPI_ROOT_OBJECT, so move the check of start_node being NULL
out of the if block.

Unfortunately, all the attempts to contact Honor have failed, they
refused to provide any technical support for Linux.

The bad DSDT table's dump could be found on GitHub [2].

DMI: HONOR FMB-P/FMB-P-PCB, BIOS 1.13 05/08/2025

Link: 1c1b57b9eb
Link: https://gist.github.com/Cryolitia/a860ffc97437dcd2cd988371d5b73ed7 [1]
Link: https://github.com/denis-bb/honor-fmb-p-dsdt [2]
Signed-off-by: Cryolitia PukNgae <cryolitia.pukngae@linux.dev>
Reviewed-by: WangYuli <wangyl5933@chinaunicom.cn>
[ rjw: Subject adjustment, changelog edits ]
Link: https://patch.msgid.link/20251125-acpica-v1-1-99e63b1b25f8@linux.dev
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/acpica/nswalk.c

@@ -169,10 +169,13 @@ acpi_ns_walk_namespace(acpi_object_type type,
 
 	if (start_node == ACPI_ROOT_OBJECT) {
 		start_node = acpi_gbl_root_node;
+	}
+
+	/* Avoid walking the namespace if the StartNode is NULL */
+
 	if (!start_node) {
 		return_ACPI_STATUS(AE_NO_NAMESPACE);
 	}
-	}
 
 	/* Null child means "get first node" */