Update demos

To build demos, run
`cmake .. -DENABLE_DEMOS=ON`

demos/src/rand_demo.c

@@ -0,0 +1,38 @@
+/*
+ *  Copyright 2014-2023 The GmSSL Project. All Rights Reserved.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the License); you may
+ *  not use this file except in compliance with the License.
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ */
+
+
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <gmssl/rand.h>
+
+
+int main(int argc, char **argv)
+{
+	uint8_t buf[1024];
+	size_t i;
+
+	if (rand_bytes(buf, 32) != 1) {
+		fprintf(stderr, "rand_bytes() failure\n");
+		return 1;
+	}
+	printf("rand_bytes() output: ");
+	for (i = 0; i < 32; i++) {
+		printf("%02x", buf[i]);
+	}
+	printf("\n");
+
+	if (rand_bytes(buf, sizeof(buf)) != 1) {
+		fprintf(stderr, "rand_bytes() failure, maybe %zu is too long\n", sizeof(buf));
+		return 1;
+	}
+
+	return 0;
+}