Assert that the timestats we need for VSL_StatSess are valid.

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@1866 d4fa192b-c00b-0410-8231-f00ffab90ce4

bin/varnishd/cache_session.c

@@ -340,6 +340,8 @@ SES_Delete(struct sess *sp)
 	AZ(sp->vcl);
 	VSL_stats->n_sess--;
 	ses_relsrcaddr(sp);
+	assert(!isnan(b->first));
+	assert(!isnan(sp->t_end));
 	VSL(SLT_StatSess, sp->id, "%s %s %.0f %ju %ju %ju %ju %ju %ju %ju",
 	    sp->addr, sp->port, sp->t_end - b->first,
 	    b->sess, b->req, b->pipe, b->pass,