Set sess to NULL after freeing it.

Found by OSS-fuzz

Bug introduced in commit 61fb59238dad6452a37ec14513fae617a4faef29

Reviewed-by: Matt Caswell <matt@openssl.org>
GH: #6235

ssl/t1_lib.c

diff --git a/ssl/t1_lib.c b/ssl/t1_lib.c
index b312a14..c076782 100644
--- a/ssl/t1_lib.c
+++ b/ssl/t1_lib.c
@@ -1384,6 +1384,7 @@
         /* Some additional consistency checks */
         if (slen != 0) {
             SSL_SESSION_free(sess);
+            sess = NULL;
             ret = SSL_TICKET_NO_DECRYPT;
             goto end;
         }