Switch some easy SSL fields to UniquePtr.

Change-Id: I982ecda5a19187708b15e8572e6d0000c22ed87c
Reviewed-on: https://boringssl-review.googlesource.com/29590
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/tls13_client.cc b/ssl/tls13_client.cc
index 1f1d4b4..dd0977a 100644
--- a/ssl/tls13_client.cc
+++ b/ssl/tls13_client.cc
@@ -328,7 +328,7 @@
       return ssl_hs_error;
     }
 
-    if (!ssl_session_is_context_valid(hs, ssl->session)) {
+    if (!ssl_session_is_context_valid(hs, ssl->session.get())) {
       // This is actually a client application bug.
       OPENSSL_PUT_ERROR(SSL,
                         SSL_R_ATTEMPT_TO_REUSE_SESSION_IN_DIFFERENT_CONTEXT);
@@ -338,7 +338,8 @@
 
     ssl->s3->session_reused = true;
     // Only authentication information carries over in TLS 1.3.
-    hs->new_session = SSL_SESSION_dup(ssl->session, SSL_SESSION_DUP_AUTH_ONLY);
+    hs->new_session =
+        SSL_SESSION_dup(ssl->session.get(), SSL_SESSION_DUP_AUTH_ONLY);
     if (!hs->new_session) {
       ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_INTERNAL_ERROR);
       return ssl_hs_error;