Ensure that `dataset` is still open when reading `tags`

[JackDunnNZ]

Previously this statement was outside the ctx context, and so dataset may already have been freed before we call dataset.tags(). In my experience, this was sporadically leading to either of the following errors:

rasterio.errors.RasterioIOError: Dataset is closed`

or

rasterio._err.ObjectNullError: Pointer 'hObject' is NULL in 'GDALGetMetadata'.

The corresponding statement for the read method was already inside the ctx context, so this was only a problem for the point method.

[vincentsarago]

good catch 🙏

[vincentsarago]

@JackDunnNZ I'm going to merge this and make a new release today 🙏

[JackDunnNZ]

Thanks!