apps/s_server.c: Add check for OPENSSL_strdup

As the potential failure of the OPENSSL_strdup(),
it should be better to check the return value and
return error if fails.

Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>

Reviewed-by: Todd Short <todd.short@me.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18595)

apps/s_server.c

@@ -1168,6 +1168,8 @@ int s_server_main(int argc, char *argv[])
         case OPT_UNIX:
             socket_family = AF_UNIX;
             OPENSSL_free(host); host = OPENSSL_strdup(opt_arg());
+            if (host == NULL)
+                goto end;
             OPENSSL_free(port); port = NULL;
             break;
         case OPT_UNLINK: