coding guidelines: comply with MISRA C:2012 Rule 21.13

MISRA C:2012 Rule 21.13 (Any value passed to a function in <ctype.h>
shall be representable as an unsigned char or be the value EOF).

Functions in <ctype.h> have undefined behavior if they are called with
any other value. Callers affected by this change are not prepared to
handle EOF anyway. The addition of these casts avoids the issue
and does not result in any performance penalty.

Signed-off-by: Abramo Bagnara <abramo.bagnara@bugseng.com>