Adding ALPN to session.

0-RTT requires matching the selected ALPN parameters against those in
the session. Stash the ALPN value in the session in TLS 1.3, so we can
recover it.

BUG=76

Change-Id: I8668b287651ae4deb0bf540c0885a02d189adee0
Reviewed-on: https://boringssl-review.googlesource.com/13845
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/tls13_server.c

diff --git a/ssl/tls13_server.c b/ssl/tls13_server.c
index 0278b50..5c9e6db 100644
--- a/ssl/tls13_server.c
+++ b/ssl/tls13_server.c
@@ -251,6 +251,17 @@
     return ssl_hs_error;
   }
 
+  /* Store the initial negotiated ALPN in the session. */
+  if (ssl->s3->alpn_selected != NULL) {
+    ssl->s3->new_session->early_alpn =
+        BUF_memdup(ssl->s3->alpn_selected, ssl->s3->alpn_selected_len);
+    if (ssl->s3->new_session->early_alpn == NULL) {
+      ssl3_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_INTERNAL_ERROR);
+      return ssl_hs_error;
+    }
+    ssl->s3->new_session->early_alpn_len = ssl->s3->alpn_selected_len;
+  }
+
   /* Incorporate the PSK into the running secret. */
   if (ssl->s3->session_reused) {
     if (!tls13_advance_key_schedule(hs, ssl->s3->new_session->master_key,