crush: fix using plain integer as NULL warning

Fixes the following sparse warnings:

net/ceph/crush/mapper.c:517:76: warning: Using plain integer as NULL pointer
net/ceph/crush/mapper.c:728:68: warning: Using plain integer as NULL pointer

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>