5 pages of text, 3 figures, corrected two typing errors: in paragraph
  1 of page 2 "p(x)=0.94 - replaced by p(x)=0.094", in paragraph 2 of page 5
  "2/3 - replaced by 2/5". Accepted for publication in Journal of Physics
  Condensed Matter in a revised form (one page added to provide some more
  informations on length distributions of CuO chains and some references added)