Initialize PRF with random data when it is enabled.

jpm

@@ -535,6 +535,16 @@
     ```
 
 int main(int argc, const char **argv) {
+
+#if defined(JANET_PRF)
+    uint8_t hash_key[JANET_HASH_KEY_SIZE];
+    if (janet_cryptorand(hash_key, JANET_HASH_KEY_SIZE) != 0) {
+        fputs("unable to initialize janet PRF hash function.\n", stderr);
+        return 1;
+    }
+    janet_init_hash_key(hash_key);
+#endif
+
     janet_init();
 
     /* Get core env */