cciss: fix cciss_revalidate panic

If you delete a logical drive, and then run BLKRRPART (e.g. via fdisk)
on a logical drive which is "after" the deleted logical drive in the h->drv[]
array, then cciss_revalidate panics because it will access the null pointer
h->drv[x] when x hits the deleted drive.

Signed-off-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Cc: stable@kernel.org
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>