kern/partition: Add sanity check after grub_strtoul() call

kern/partition: Add sanity check after grub_strtoul() call

The current code incorrectly assumes that both the input and the values
returned by grub_strtoul() are always valid which can lead to potential
errors. This fix ensures proper validation to prevent any unintended issues.

Fixes: CID 473843
Signed-off-by: Lidong Chen <lidong.chen@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Gbp-Pq: Topic cve-2025-jan
Gbp-Pq: Name kern-partition-Add-sanity-check-after-grub_strtoul-call.patch