row indices are not sorted within columns

```r
library(anndata)

ad <- AnnData(
  X = Matrix::rsparsematrix(10, 10, .5)
)
ad$X
ix <- sample.int(10)

ad[ix,]$X
```

Results in
```
Error in validObject(.Object) : 
  invalid class “dgCMatrix” object: row indices are not sorted within columns
```