initialise FP_NR<longdouble> to zero
initialise FP_NR<longdouble> to zero

dstehle/fplll,shi-bai/fplll,cr-marcstevens/fplll,fplll/fplll,cr-marcstevens/fplll,fplll/fplll,shi-bai/fplll,fplll/fplll,fplll/fplll,cr-marcstevens/fplll,dstehle/fplll,shi-bai/fplll,fplll/fplll,shi-bai/fplll,dstehle/fplll,cr-marcstevens/fplll