Add some more consistency checks in tls_decrypt_ticket.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2704)

ssl/t1_lib.c

diff --git a/ssl/t1_lib.c b/ssl/t1_lib.c
index 140b1f1..f13c0ad 100644
--- a/ssl/t1_lib.c
+++ b/ssl/t1_lib.c
@@ -1315,6 +1315,11 @@
     sess = d2i_SSL_SESSION(NULL, &p, slen);
     OPENSSL_free(sdec);
     if (sess) {
+        /* Some additional consistency checks */
+        if (p != sdec + slen || sess->session_id_length != 0) {
+            SSL_SESSION_free(sess);
+            return 2;
+        }
         /*
          * The session ID, if non-empty, is used by some clients to detect
          * that the ticket has been accepted. So we copy it to the session