If alloc_disk fails in pcd_init_units, cd->disk & pi are empty, we need
to check if cd->disk is null in pcd_detect.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>