[gimp] app: get_set_error() with domain == 0 is forbidden.

[Jehan Pagès <jehanp src gnome org>]

commit 9da25e6261a0e27e2bedbbe0649349bce2c9fbe1
Author: Jehan <jehan girinstud io>
Date:   Sun Dec 31 01:46:34 2017 +0100

    app: get_set_error() with domain == 0 is forbidden.
    
    That makes critical errors:
    > GLib-CRITICAL **: g_error_new_literal: assertion 'domain != 0' failed

 app/core/gimpimage-merge.c |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)
---
diff --git a/app/core/gimpimage-merge.c b/app/core/gimpimage-merge.c
index 41dcc14..a26724b 100644
--- a/app/core/gimpimage-merge.c
+++ b/app/core/gimpimage-merge.c
@@ -259,14 +259,14 @@ gimp_image_merge_down (GimpImage      *image,
         {
           if (gimp_viewable_get_children (GIMP_VIEWABLE (layer)))
             {
-              g_set_error_literal (error, 0, 0,
+              g_set_error_literal (error, GIMP_ERROR, GIMP_FAILED,
                                    _("Cannot merge down to a layer group."));
               return NULL;
             }
 
           if (gimp_item_is_content_locked (GIMP_ITEM (layer)))
             {
-              g_set_error_literal (error, 0, 0,
+              g_set_error_literal (error, GIMP_ERROR, GIMP_FAILED,
                                    _("The layer to merge down to is locked."));
               return NULL;
             }
@@ -278,7 +278,7 @@ gimp_image_merge_down (GimpImage      *image,
 
   if (! merge_list)
     {
-      g_set_error_literal (error, 0, 0,
+      g_set_error_literal (error, GIMP_ERROR, GIMP_FAILED,
                            _("There is no visible layer to merge down to."));
       return NULL;
     }