Assert that alpn_selected is NULL before we assign it

The alpn_selected value in the session should be NULL before we first
populate it if this is a new session. We assert to make sure it is.
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5715)

ssl/statem/extensions_clnt.c

@@ -1679,7 +1679,15 @@ int tls_parse_stoc_alpn(SSL *s, PACKET *pkt, unsigned int context, X509 *x,
         s->ext.early_data_ok = 0;
     }
     if (!s->hit) {
-        /* If a new session then update it with the selected ALPN */
+        /*
+         * This is a new session and so alpn_selected should have been
+         * initialised to NULL. We should update it with the selected ALPN.
+         */
+        if (!ossl_assert(s->session->ext.alpn_selected == NULL)) {
+            SSLfatal(s, SSL_AD_INTERNAL_ERROR, SSL_F_TLS_PARSE_STOC_ALPN,
+                     ERR_R_INTERNAL_ERROR);
+            return 0;
+        }
         s->session->ext.alpn_selected =
             OPENSSL_memdup(s->s3->alpn_selected, s->s3->alpn_selected_len);
         if (s->session->ext.alpn_selected == NULL) {

ssl/statem/statem_srvr.c

@@ -2100,7 +2100,17 @@ int tls_handle_alpn(SSL *s)
                 s->ext.early_data_ok = 0;
 
                 if (!s->hit) {
-                    /* If a new session update it with the new ALPN value */
+                    /*
+                     * This is a new session and so alpn_selected should have
+                     * been initialised to NULL. We should update it with the
+                     * selected ALPN.
+                     */
+                    if (!ossl_assert(s->session->ext.alpn_selected == NULL)) {
+                        SSLfatal(s, SSL_AD_INTERNAL_ERROR,
+                                 SSL_F_TLS_HANDLE_ALPN,
+                                 ERR_R_INTERNAL_ERROR);
+                        return 0;
+                    }
                     s->session->ext.alpn_selected = OPENSSL_memdup(selected,
                                                                    selected_len);
                     if (s->session->ext.alpn_selected == NULL) {