ACPICA: Avoid walking the ACPI Namespace if it is not there

ACPICA: Avoid walking the ACPI Namespace if it is not there

ACPICA commit b1c3656ef4950098e530be68d4b589584f06cddc

Prevent acpi_ns_walk_namespace() from crashing when called with
start_node equal to ACPI_ROOT_OBJECT if the Namespace has not been
instantiated yet and acpi_gbl_root_node is NULL.

For instance, this can happen if the kernel is run with "acpi=off"
in the command line.

Link: https://github.com/acpica/acpica/commit/b1c3656ef4950098e530be68d4b589584f06cddc
Link: https://lore.kernel.org/linux-acpi/CAJZ5v0hJWW_vZ3wwajE7xT38aWjY7cZyvqMJpXHzUL98-SiCVQ@mail.gmail.com/
Reported-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>