disk/diskfilter: Check calloc() result for NULL

disk/diskfilter: Check calloc() result for NULL

With wildly corrupt inputs, we can end up trying to calloc a very
large amount of memory, which will fail and give us a NULL pointer.
We need to check that to avoid a crash. (And, even if we blocked
such inputs, it is good practice to check the results of allocations
anyway.)

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>