Memory leak on dataset error

## Description

When error is encountered opening a gdal dataset, an error is returned, but the `GDALDataset` is not cleaned up, leaving C memory still allocated.

https://github.com/airbusgeo/godal/blob/v0.0.15/godal.go#L1545-L1550

If opening a lot of datasets with errors, the memory use will continue building up.
