caif: fix NULL pointer check

[ Upstream commit c66b9b7d365444b433307ebb18734757cb668a02 ]

Reported-by: <rucsoftsec@gmail.com>
Resolves-bug: http://bugzilla.kernel.org/show_bug?44441
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>