cciss: Make cciss_seq_show handle holes in the h->drv[] array

It is possible (and expected) for there to be holes in the h->drv[]
array, that is, some elements may be NULL pointers. cciss_seq_show
needs to be made aware of this possibility to avoid an Oops.

To reproduce the Oops which this fixes:

1) Create two "arrays" in the Array Configuratino Utility and
several logical drives on each array.
2) cat /proc/driver/cciss/cciss* in an infinite loop
3) delete some of the logical drives in the first "array."

Signed-off-by: Stephen M. Cameron <[e-mail]>
Cc: [e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/cciss.c

···
 	if (*pos > h->highest_lun)
 		return 0;
 
+	if (drv == NULL) /* it's possible for h->drv[] to have holes. */
+		return 0;
+
 	if (drv->heads == 0)
 		return 0;