lib:util: Fix printing hex‐escaped characters

lib:util: Fix printing hex‐escaped characters

A signed char, passed to snprintf(), will be promoted to an ‘int’, and
then interpreted (according to the format string) as an ‘unsigned int’.
Any negative values passed in will thus be interpreted as large unsigned
values, too large to be represented in the two characters allocated for
them. In practice, they will always be represented as ‘\xFF’.

Cast these characters to ‘unsigned char’, and use the appropriate length
modifier for that type.

Signed-off-by: Jo Sutton <josutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>