Rewrite timeout computation in a way that is less prone to overflow.

(Problem reported by Peter Sylvester.)

ssl/ssl_sess.c

@@ -389,7 +389,7 @@ int ssl_get_prev_session(SSL *s, unsigned char *session_id, int len)
 	CRYPTO_add(&ret->references,1,CRYPTO_LOCK_SSL_SESSION);
 #endif
 
-	if ((long)(ret->time+ret->timeout) < (long)time(NULL)) /* timeout */
+	if (ret->timeout < (long)(time(NULL) - ret->time)) /* timeout */
 		{
 		s->ctx->stats.sess_timeout++;
 		/* remove it from the cache */