MetaBackendNative: Initialize GError pointer

Initialize the GError pointer used when creating the renderer. If an
error occurs, the error is expected to be NULL, otherwise it'll
misinterpreted as already set.

https://bugzilla.gnome.org/show_bug.cgi?id=773629

src/backends/native/meta-backend-native.c

@@ -290,7 +290,7 @@ meta_backend_native_create_renderer (MetaBackend *backend)
   MetaBackendNativePrivate *priv =
     meta_backend_native_get_instance_private (native);
   int kms_fd;
-  GError *error;
+  GError *error = NULL;
   MetaRendererNative *renderer_native;
 
   kms_fd = meta_launcher_get_kms_fd (priv->launcher);