ACPI: fix NULL bug for HID/UID string

acpi_device->pnp.hardware_id and unique_id are now allocated pointers,
replacing the previous arrays.  acpi_device_install_notify_handler()
oopsed on the NULL hid when probing the video device, and perhaps other
uses are vulnerable too.  So initialize those pointers to empty strings
when there is no hid or uid.  Also, free hardware_id and unique_id when
when acpi_device is going to be freed.

http://bugzilla.kernel.org/show_bug.cgi?id=14096



Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>