misc: sgi-xp: remove meaningless null check before kfree

kfree has taken null pointer into account. so check the null pointer
before kfree is meaningless.

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Acked-by: Robin Holt <robinmholt@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>