PtrAttribute always returns nil (and might segfault)

[hsjsstn]

addressing.go passes p instead of &p to cuPointerGetAttribute:

var p unsafe.Pointer
C.cuPointerGetAttribute(p, a, devPtr)  // p is nil here
return p, nil                          // so this is always nil

The API writes the result into the address you give it, so passing nil means nothing gets written
and on some drivers it'll segfault.

Fix is just:

C.cuPointerGetAttribute(unsafe.Pointer(&p), a, devPtr)