ACPICA: fixup after acpi_get_object_info() change

Commit 15b8dd53

 changed info->hardware_id from a static array to
a pointer.  If hardware_id is non-NULL, it points to a NULL-terminated
string, so we don't need to terminate it explicitly.  However, it may
be NULL; in that case, we *can't* add a NULL terminator.

This causes a NULL pointer dereference oops for devices without _HID.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
CC: Lin Ming <ming.m.lin@intel.com>
CC: Bob Moore <robert.moore@intel.com>
CC: Gary Hade <garyhade@us.ibm.com>
Signed-off-by: Len Brown <len.brown@intel.com>