Fix a -Wsign-compare warning

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2940)

apps/s_client.c

@@ -174,7 +174,7 @@ static unsigned int psk_client_cb(SSL *ssl, const char *hint, char *identity,
                    psk_key);
         return 0;
     }
-    if (key_len > max_psk_len) {
+    if (max_psk_len > INT_MAX || key_len > (long)max_psk_len) {
         BIO_printf(bio_err,
                    "psk buffer of callback is too small (%d) for key (%ld)\n",
                    max_psk_len, key_len);