Avoid sending alerts after shutdown

Fixes #11388

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15159)

ssl/s3_msg.c

@@ -52,6 +52,8 @@ int ssl3_send_alert(SSL *s, int level, int desc)
                                           * protocol_version alerts */
     if (desc < 0)
         return -1;
+    if (s->shutdown & SSL_SENT_SHUTDOWN && desc != SSL_AD_CLOSE_NOTIFY)
+        return -1;
     /* If a fatal one, remove from cache */
     if ((level == SSL3_AL_FATAL) && (s->session != NULL))
         SSL_CTX_remove_session(s->session_ctx, s->session);