kprintf: explicitly disable wchar support

This might get implemented later, even though
there is no real reason why we would need wchar
support in the kernel as it is UTF-8 native

kernel/kprintf.c

@@ -312,17 +312,12 @@ void parse_fmt_sequence(struct fmt_sequence *sequence, const char **restrict pos
 
 static ssize_t render_c(const struct fmt_sequence *sequence, va_list *ap)
 {
-	ssize_t ret;
-
-	if (sequence->length_modifier == LENGTH_L) {
-		wchar_t val = (wchar_t)va_arg(*ap, wint_t);
-		ret = printer->write(printer, &val, sizeof(val));
-	} else {
-		char val = (char)va_arg(*ap, int);
-		ret = printer->write(printer, &val, sizeof(val));
-	}
+	/* we don't support wchars until we have a UTF-8 encoder */
+	if (sequence->length_modifier != LENGTH_DEFAULT)
+		return -1;
 
-	return ret;
+	char val = (char)va_arg(*ap, int);
+	return printer->write(printer, &val, sizeof(val));
 }
 
 static ssize_t render_s(const struct fmt_sequence *sequence, va_list *ap)
@@ -331,7 +326,13 @@ static ssize_t render_s(const struct fmt_sequence *sequence, va_list *ap)
 	 * the string is a wchar_t if LENGTH_L is set, but that would require
 	 * a full UTF-8 encoder which i won't write in the near future.  Cope.
 	 */
+	if (sequence->length_modifier != LENGTH_DEFAULT)
+		return -1;
+
 	const char *s = va_arg(*ap, char *);
+	if (s == nil)
+		return write_asciz("(null)");
+
 	if (sequence->max_precision)
 		return write_bytes(s, strnlen(s, sequence->max_precision));
 	else