Make sure to use unsigned char for is*() functions

On some platforms, the implementation is such that a signed char
triggers a warning when used with is*() functions.  On others, the
behavior is outright buggy when presented with a char that happens
to get promoted to a negative integer.

The safest thing is to cast the char that's used to an unsigned char.

Reviewed-by: Andy Polyakov <appro@openssl.org>

apps/s_server.c

@@ -1380,7 +1380,7 @@ int s_server_main(int argc, char *argv[])
         case OPT_PSK:
 #ifndef OPENSSL_NO_PSK
             for (p = psk_key = opt_arg(); *p; p++) {
-                if (isxdigit(*p))
+                if (isxdigit(_UC(*p)))
                     continue;
                 BIO_printf(bio_err, "Not a hex number '%s'\n", *argv);
                 goto end;