[gimp] app: gimp_image_merge_down() on a floating selection crashes GIMP.

[Jehan Pagès <jehanp src gnome org>]

commit cc47597aaef295ba5f6196fe8c27f7a7fa40814f
Author: Jehan <jehan girinstud io>
Date:   Sat Dec 30 23:26:55 2017 +0100

    app: gimp_image_merge_down() on a floating selection crashes GIMP.
    
    Forbid merging down a floating selection.
    Probably we can do something better. But that's a first step.

 app/core/gimpimage-merge.c |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)
---
diff --git a/app/core/gimpimage-merge.c b/app/core/gimpimage-merge.c
index e4d0f94..138845e 100644
--- a/app/core/gimpimage-merge.c
+++ b/app/core/gimpimage-merge.c
@@ -239,6 +239,13 @@ gimp_image_merge_down (GimpImage      *image,
   g_return_val_if_fail (GIMP_IS_CONTEXT (context), NULL);
   g_return_val_if_fail (error == NULL || *error == NULL, NULL);
 
+  if (gimp_layer_is_floating_sel (current_layer))
+    {
+      g_set_error_literal (error, GIMP_ERROR, GIMP_FAILED,
+                           _("Cannot merge down a floating selection."));
+      return NULL;
+    }
+
   for (list = gimp_item_get_container_iter (GIMP_ITEM (current_layer));
        list;
        list = g_list_next (list))