Fix perfect_foresight_problem.cc for case without leaded variables

The file contains the error message

mexErrMsgTxt("M_.lead_lag_incidence should be a real dense matrix with 2+M_.maximum_endo_lag rows and M_.endo_nbr columns");

but M_.lead_lag_incidence is 1 + M_.maximum_endo_lag +M_.maximum_endo_lead, not 2+M_.maximum_endo_lag

test4.mod should trigger the issue.