update generate_probes() to support -1 as last step
update generate_probes() to support -1 as last step

sfepy/sfepy,BubuLK/sfepy,RexFuzzle/sfepy,sfepy/sfepy,BubuLK/sfepy,RexFuzzle/sfepy,lokik/sfepy,vlukes/sfepy,lokik/sfepy,rc/sfepy,BubuLK/sfepy,lokik/sfepy,lokik/sfepy,vlukes/sfepy,rc/sfepy,rc/sfepy,RexFuzzle/sfepy,sfepy/sfepy,vlukes/sfepy,RexFuzzle/sfepy