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>