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.
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