square bracket accessor exception when exceeding dims of data frame

[MattNapsAlot]

Modify the method to behave like a data frame:

> dd <- data.frame(diag(3))
> dd[4,]
   X1 X2 X3
NA NA NA NA
> x <- BigDataFrame(data=dd)
> x[4,]
Error in HDF5Execute("readdata", filename = filename, path = path, intdata = integer(),  : 
  HDF5Dataset::ReadData() Starting row index exceeds the number of rows in the dataset.