agp/hp: fixup hp agp after ACPI changes

Commit 15b8dd53

 changed the string in info->hardware_id from a static
array to a pointer and added a length field.  But instead of changing
"sizeof(array)" to "length", we changed it to "sizeof(length)" (== 4),
which corrupts the string we're trying to null-terminate.

We no longer even need to null-terminate the string, but we *do* need to
check whether we found a HID.  If there's no HID, we used to have an empty
array, but now we have a null pointer.

The combination of these defects causes this oops:

  Unable to handle kernel NULL pointer dereference (address 0000000000000003)
  modprobe[895]: Oops 8804682956800 [1]
  ip is at zx1_gart_probe+0xd0/0xcc0 [hp_agp]

  http://marc.info/?l=linux-ia64&m=126264484923647&w=2

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Reported-by: Émeric Maschino <emeric.maschino@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>