[gimp] Bug 792744 - Gimp crashes attempting to merge down a hidden layer.

[Jehan Pagès <jehanp src gnome org>]

commit b9577a783d2d43fbe2eb65523f1199ccc1a66972
Author: Jehan <jehan girinstud io>
Date:   Sun Jan 21 19:49:19 2018 +0100

    Bug 792744 - Gimp crashes attempting to merge down a hidden layer.
    
    Don't use g_assert(). Instead use g_return_val_if_fail().
    This commit therefore does not fix the actual bug, but at least it does
    not crash. GIMP simply outputs a warning upon trying to merge down a
    hidden layer. The actual fix will follow later.

 app/core/gimpimage-merge.c |   16 ++++++++--------
 1 files changed, 8 insertions(+), 8 deletions(-)
---
diff --git a/app/core/gimpimage-merge.c b/app/core/gimpimage-merge.c
index 54c72e7..8169358 100644
--- a/app/core/gimpimage-merge.c
+++ b/app/core/gimpimage-merge.c
@@ -449,7 +449,14 @@ gimp_image_merge_layers (GimpImage     *image,
   g_return_val_if_fail (GIMP_IS_IMAGE (image), NULL);
   g_return_val_if_fail (GIMP_IS_CONTEXT (context), NULL);
 
-  parent = gimp_layer_get_parent (merge_list->data);
+  top_layer = merge_list->data;
+  parent    = gimp_layer_get_parent (top_layer);
+
+  /*  Build our graph inside the top-layer's parent node  */
+  source_node = gimp_filter_get_node (GIMP_FILTER (top_layer));
+  node = gegl_node_get_parent (source_node);
+
+  g_return_val_if_fail (node, NULL);
 
   /*  Get the layer extents  */
   x1 = y1 = 0;
@@ -519,7 +526,6 @@ gimp_image_merge_layers (GimpImage     *image,
   if ((x2 - x1) == 0 || (y2 - y1) == 0)
     return NULL;
 
-  top_layer    = merge_list->data;
   bottom_layer = layer;
 
   flatten_node = NULL;
@@ -589,12 +595,6 @@ gimp_image_merge_layers (GimpImage     *image,
    */
   (void) gimp_projectable_get_graph (GIMP_PROJECTABLE (image));
 
-  /*  Build our graph inside the top-layer's parent node  */
-  source_node = gimp_filter_get_node (GIMP_FILTER (top_layer));
-
-  node = gegl_node_get_parent (source_node);
-  g_assert (node != NULL);
-
   offset_node = gegl_node_new_child (node,
                                      "operation", "gegl:translate",
                                      "x",         (gdouble) -x1,