Fix nid assignment in ASN1_STRING_TABLE_add

CLA: trivial
Reviewed-by: NKurt Roeckx <kurt@roeckx.be>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3934)

crypto/asn1/a_strnid.c

 /*
- * Copyright 1999-2016 The OpenSSL Project Authors. All Rights Reserved.
+ * Copyright 1999-2017 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
@@ -167,6 +167,7 @@ static ASN1_STRING_TABLE *stable_get(int nid)
         rv->mask = tmp->mask;
         rv->flags = tmp->flags | STABLE_FLAGS_MALLOC;
     } else {
+        rv->nid = nid;
         rv->minsize = -1;
         rv->maxsize = -1;
         rv->flags = STABLE_FLAGS_MALLOC;