Merge bitcoin-core/secp256k1#1212: Prevent dead-store elimination when clearing secrets in examples

5660c13755 prevent optimization in algorithms (Harshil Jani)

Pull request description:

  Signed-off-by: Harshil Jani <harshiljani2002@gmail.com>

ACKs for top commit:
  sipa:
    utACK 5660c13755
  real-or-random:
    utACK 5660c13755

Tree-SHA512: 90024b7445c04e18a88af4099fc1ac6d1b9b2309b88dd22ae2b1f50aed7bac28b2c180cc28e1a95d5e9ec94b4c4adc44b9ada1477e6abe8efae7884c2382645c

examples/schnorr.c

@@ -15,7 +15,7 @@
 #include <secp256k1_extrakeys.h>
 #include <secp256k1_schnorrsig.h>
 
-#include "random.h"
+#include "examples_util.h"
 
 int main(void) {
     unsigned char msg[12] = "Hello World!";
@@ -149,9 +149,8 @@ int main(void) {
      * example through "out of bounds" array access (see Heartbleed), Or the OS
      * swapping them to disk. Hence, we overwrite the secret key buffer with zeros.
      *
-     * TODO: Prevent these writes from being optimized out, as any good compiler
+     * Here we are preventing these writes from being optimized out, as any good compiler
      * will remove any writes that aren't used. */
-    memset(seckey, 0, sizeof(seckey));
-
+    secure_erase(seckey, sizeof(seckey));
     return 0;
 }