Don't acknowledge a servername following warning alert in servername cb

If the servername cb decides to send back a warning alert then the
handshake continues, but we should not signal to the client that the
servername has been accepted.
Reviewed-by: NBen Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/10018)

ssl/statem/extensions.c

@@ -1009,6 +1009,7 @@ static int final_server_name(SSL *s, unsigned int context, int sent)
         /* TLSv1.3 doesn't have warning alerts so we suppress this */
         if (!SSL_IS_TLS13(s))
             ssl3_send_alert(s, SSL3_AL_WARNING, altmp);
+        s->servername_done = 0;
         return 1;
 
     case SSL_TLSEXT_ERR_NOACK: