Niels De Graef authored Feb 12, 2019 and Niels De Graef committed Feb 12, 2019

This means we need to make sure we don't accidentally free the provided
source GError (which automatically happens with `g_autoptr`), so use
`g_steal_pointer()`.

This fixes an issue where, when launched in a bubblewrap environment
(such as the one provided by Buildstream), mutter would give the
following warning message:

```
mutter-WARNING **: 8:31:35:069: Can't initialize KMS backend: (null)
```

... which isn't that useful when trying to debug the actual issue.