The double polymeric chain of catena-poly[(μ2-6-bromopyridine-3-carboxylato-κ2O,O′) (6-bromopyridine-3-carboxylato-κ2O,O′) (μ2-1,2-bis(4-pyridyl)ethylene-κ2N:N′)cobalt(II)], C24H16CoBr2N4O4