9158fe02a2b2caa7437229180e65ac48b5b47473 Initialize stock1_filter_step_ahead to prevent crash in Octave (cherry picked from commit cac48d2e8099d1e516cfc0753d49ea4b9cd0420c)

================================ DYNARE MAKE CHECK MATLAB RESULTS ================================ | TOTAL: 217 | PASS: 215 | FAIL: 0 | XFAIL: 2 | XPASS: 0

================================ DYNARE MAKE CHECK OCTAVE RESULTS ================================ | TOTAL: 215 | PASS: 213 | FAIL: 0 | XFAIL: 2 | XPASS: 0

A full log can be found on karaba in '/tmp/dynare-4.4-check-beXmFriTa4.log'.