gsum on int64 ignores na.rm=FALSE

```r
DT = data.table(id=c(rep(1L,3), rep(2L, 3)), v=bit64::as.integer64(c(1:3, NA, 5:6)))
DT[2:6, sum(v), id]
#      id    V1
#   <int> <i64>
#1:     1     5
#2:     2    11
DT[, "v" := as.integer(v)]
#      id    V1
#   <int> <int>
#1:     1     5
#2:     2    NA
```
might be related to #1647