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/handshake_server.cc b/ssl/handshake_server.cc
index edbe617..a0674d1 100644
--- a/ssl/handshake_server.cc
+++ b/ssl/handshake_server.cc
@@ -600,7 +600,7 @@
   if (session) {
     // Use the old session.
     hs->ticket_expected = renew_ticket;
-    ssl->session = session.release();
+    ssl->session = std::move(session);
     ssl->s3->session_reused = true;
   } else {
     hs->ticket_expected = tickets_supported;
@@ -713,8 +713,8 @@
   }
 
   const SSL_SESSION *session = hs->new_session.get();
-  if (ssl->session != NULL) {
-    session = ssl->session;
+  if (ssl->session != nullptr) {
+    session = ssl->session.get();
   }
 
   ScopedCBB cbb;
@@ -1419,7 +1419,8 @@
     } else {
       // We are renewing an existing session. Duplicate the session to adjust
       // the timeout.
-      session_copy = SSL_SESSION_dup(ssl->session, SSL_SESSION_INCLUDE_NONAUTH);
+      session_copy =
+          SSL_SESSION_dup(ssl->session.get(), SSL_SESSION_INCLUDE_NONAUTH);
       if (!session_copy) {
         return ssl_hs_error;
       }