um: don't compare a pointer to 0

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Link: http://lkml.kernel.org/r/1356030701-16284-27-git-send-email-sasha.levin@oracle.com


Cc: Richard Weinberger <richard@nod.at>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>