[SeaBIOS] PATCH: properly cast c value in memset

[Leah Rowe]

Hi

Attached is a patch I did earlier, while working on something else. I
noticed that SeaBIOS's memset() implementation casts the object "s" to
(char *), which is technically incorrect; see:

https://pubs.opengroup.org/onlinepubs/9699919799/functions/memset.html

It says:

The memset() function shall copy c (converted to an unsigned char) into
each of the first n bytes of the object pointed to by s.

The attached patch fixes this, by casting c to unsigned char, and
accordingly, casts void *s to (unsigned char *)

The current cast does (char *) which is technically incorrect, because
whether char is signed or unsigned is implementation-defined, so you
have to explicitly state signed or unsigned when signedness matters.