Fix inaccurate comments in bn_prime.c

As well as a coding style nit is fixed.
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NBernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/3763)

crypto/bn/bn_prime.c

 /*
- * WARNING: do not edit!
- * Generated by crypto/bn/bn_prime.pl
- * Copyright 1995-2016 The OpenSSL Project Authors. All Rights Reserved.
+ * Copyright 1995-2017 The OpenSSL Project Authors. All Rights Reserved.
  *
  * Licensed under the OpenSSL license (the "License").  You may not use
  * this file except in compliance with the License.  You can obtain a copy
@@ -80,7 +78,7 @@ int BN_generate_prime_ex(BIGNUM *ret, int bits, int safe,
         goto err;
     BN_CTX_start(ctx);
     t = BN_CTX_get(ctx);
-    if (!t)
+    if (t == NULL)
         goto err;
  loop:
     /* make a random number and set the top and bottom bits */