Avoid questionable use of the value of a pointer

that refers to space
deallocated by a call to the free function in tls_decrypt_ticket.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from #2897)
(cherry picked from commit 13ed1af)

ssl/t1_lib.c

@@ -3577,10 +3577,11 @@ static int tls_decrypt_ticket(SSL *s, const unsigned char *etick,
     p = sdec;
 
     sess = d2i_SSL_SESSION(NULL, &p, slen);
+    slen -= p - sdec;
     OPENSSL_free(sdec);
     if (sess) {
         /* Some additional consistency checks */
-        if (p != sdec + slen || sess->session_id_length != 0) {
+        if (slen != 0 || sess->session_id_length != 0) {
             SSL_SESSION_free(sess);
             return 2;
         }