fix _make_iterator() for less data than n_row * n_col
fix _make_iterator() for less data than n_row * n_col

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