avoid potential UB when using isprint()

all the ctype.h functions' argument must be representable as an unsigned
char or as EOF, otherwise the behavior is undefined.

ref. https://git.suckless.org/st/commit/af3bb68add1c40d19d0dee382009e21b0870a38f.html

st.c

@@ -397,7 +397,7 @@ static const char base64_digits[] = {
 char
 base64dec_getc(const char **src)
 {
-	while (**src && !isprint(**src))
+	while (**src && !isprint((unsigned char)**src))
 		(*src)++;
 	return **src ? *((*src)++) : '=';  /* emulate padding if string ends */
 }