fixing bug in cost_vary_prob()
fixing bug in cost_vary_prob()

dstehle/fplll,cr-marcstevens/fplll,fplll/fplll,cr-marcstevens/fplll,fplll/fplll,dstehle/fplll,fplll/fplll,fplll/fplll,fplll/fplll,cr-marcstevens/fplll,dstehle/fplll,cr-marcstevens/fplll