Fix for negative return value from SSL_CTX_sess_accept()

Fixes #13183

From the original issue report, before this commit, on master and on
1.1.1, the issue can be detected with the following steps:

- Start with a default SSL_CTX, initiate a TLS 1.3 connection with SNI,
  "Accept" count of default context gets incremented
- After servername lookup, "Accept" count of default context gets
  decremented and that of SNI context is incremented
- Server sends a "Hello Retry Request"
- Client sends the second "Client Hello", now again "Accept" count of
  default context is decremented. Hence giving a negative value.

This commit fixes it by adding a check on `s->hello_retry_request` in
addition to `SSL_IS_FIRST_HANDSHAKE(s)`, to ensure the counter is moved
only on the first ClientHello.

CLA: trivial

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from #13297)

ssl/statem/extensions.c

@@ -966,7 +966,8 @@ static int final_server_name(SSL *s, unsigned int context, int sent)
      * context, to avoid the confusing situation of having sess_accept_good
      * exceed sess_accept (zero) for the new context.
      */
-    if (SSL_IS_FIRST_HANDSHAKE(s) && s->ctx != s->session_ctx) {
+    if (SSL_IS_FIRST_HANDSHAKE(s) && s->ctx != s->session_ctx
+		    && s->hello_retry_request == SSL_HRR_NONE) {
         tsan_counter(&s->ctx->stats.sess_accept);
         tsan_decr(&s->session_ctx->stats.sess_accept);
     }