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

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

When the format string, fmt0, includes a positional argument
grub_strtoul() or grub_strtoull() is called to extract the argument
position. However, the returned argument position isn't fully validated.
If the format is something like "%0$x" then these functions return
0 which leads to an underflow in the calculation of the args index, curn.
The fix is to add a check to ensure the extracted argument position is
greater than 0 before computing curn. Additionally, replace one
grub_strtoull() with grub_strtoul() and change curn type to make code
more correct.

Fixes: CID 473841
Signed-off-by: Lidong Chen <lidong.chen@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>