stochastic_solvers.m: set dr.state_var for purely backwards models at order=1

Subsequent codes may need it. Closes Dynare/dynare#1703

matlab/stochastic_solvers.m

@@ -216,6 +216,7 @@ if M_.maximum_endo_lead == 0
     % backward models: simplified code exist only at order == 1
     if local_order == 1
         [k1,~,k2] = find(kstate(:,4));
+        dr.state_var = k2;
         dr.ghx(:,k1) = -b\jacobia_(:,k2);
         if M_.exo_nbr
             dr.ghu =  -b\jacobia_(:,nz+1:end);