Ensure we never return 1 from sodium_init() onload

sodium_init() will return 0 on success, -1 on failure, and 1 if sodium is
already loaded and initialized (which is not an error). In the case
where libsodium is already initialized and the system is restarted we
may return 1 from onload nif function resulting in a crash.

- change the call to sodium_init() to check for an error return (-1) and
  return -1 explicitly in this case, otherwise always return zero at the
  end of our onload function.

c_src/enacl_nif.c

@@ -41,7 +41,11 @@ static int enacl_crypto_load(ErlNifEnv *env, void **priv_data,
     return -1;
   }
 
-  return sodium_init();
+  if (sodium_init() == -1) {
+    return -1;
+  }
+
+  return 0;
 }
 
 /* GENERAL ROUTINES