:= assignment to under-allocated S4 slot fails

Another offshoot of #6575, but this issue also affects 1.16.4, so fixing it will be a separate bugfix.

```r
setClass("DataTable", slots=c(x="data.table"))
options(datatable.alloccol=0L)
DT = new("DataTable", x=data.table(a=1))
DT@x[, b := 2L]
DT@x$b
# NULL
```