sgi-xpc: Remove NULL pointer dereference.

If the bte copy fails, the attempt to retrieve payloads merely returns a
null pointer deref and not NULL as was expected.

Signed-off-by: Robin Holt <holt@sgi.com>
Signed-off-by: Dean Nelson <dcn@sgi.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>