82d43818f5f276fb066bc0b45c31037a378d92f2 Fix issue with purely forward deterministic models with lags on exogenous.

================================ DYNARE MAKE CHECK MATLAB RESULTS ================================ | TOTAL: 439 | PASS: 437 | FAIL: 0 | XFAIL: 2 | XPASS: 0

================================ DYNARE MAKE CHECK OCTAVE RESULTS ================================ | TOTAL: 433 | PASS: 431 | FAIL: 0 | XFAIL: 2 | XPASS: 0

A full log can be found on karaba in '/tmp/dynare-4.4-check-T1hOkh4LFs.log'.