lib: fix pointer of type 'void *' used in arithmetic

Using "void *" in arithmetic causes errors with strict compiler settings:
"error: pointer of type 'void *' used in arithmetic [-Werror=pointer-arith]"

Avoid these by calculating on "char *" where 1-byte data size is assumed.

Signed-off-by: Jukka Laitinen <jukkax@ssrc.tii.ae>
Reviewed-by: Dong Du <Dd_nirvana@sjtu.edu.cn>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>