As of now, if one tries to use GListModel APIs on PeasExtensionSet,
specifically g_list_model_get_item(), it'll generate a warning and
potentially a crash due to NULL pointers.

See, the 'extensions' field of PeasExtensionSet is a GQueue that
holds ExtensionItem elements. ExtensionItems are not GObjects, but
they do hold a reference to the instantiated extension which must
be a GObject.

The GListModel implementation of PeasExtensionSet tries to call
g_object_ref() on an ExtensionItem pointer, which fails and returns
NULL. The appropriate return value would be to call g_object_ref()
on the 'exten' field of the ExtensionItem at that position.

Do just that.