Sporadic error in kalmanSmooth (PR#6738)

Defining the following: 

set.seed(123)

kalmanTorture <- function(iter) {
  x <- arima.sim(model = list(ar=0.9, ma=0.5),n=150 )
  x[10:20] <- NA
  mod <- arima(x, order=c(1,0,1) )
  for (i in 1:iter) {
     smooth <- KalmanSmooth(x, mod=mod$model)$smooth
     if (any(is.na(smooth))) stop("NA on iteration ",i) }
}

and running in R1.9.0beta (equal in R1.8.1), windows XP, precompiled 
binaries:

kalmanTorture(1000)

Error in kalmanTorture(1000) : NA on iteration 147

kalmanTorture(1000)

Error in kalmanTorture(1000) : NA on iteration 3

kalmanTorture(1000)

Error in kalmanTorture(1000) : NA on iteration 2

kalmanTorture(1000)

Error in kalmanTorture(1000) : NA on iteration 3

kalmanTorture(1000)

Error in kalmanTorture(1000) : NA on iteration 1

kalmanTorture(1000)

Error in kalmanTorture(1000) : NA on iteration 4

kalmanTorture(1000)

Error in kalmanTorture(1000) : NA on iteration 1
.
.
.
Starting a new session, using the same set.seed(123)
gives another sequence of iteration numbers where failing.