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

[Michael Natterer <mitch src gnome org>]

commit af2cb761de23945c673d7f84af636f4092a3cbe6
Author: Michael Natterer <mitch gimp org>
Date:   Sun Jan 21 23:58:42 2018 +0100

    Bug 792744 - Gimp crashes attempting to merge down a hidden layer
    
    gimp_image_merge_down(): return an error if the current layer is not
    visible.

 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 8169358..7be33ca 100644
--- a/app/core/gimpimage-merge.c
+++ b/app/core/gimpimage-merge.c
@@ -246,6 +246,13 @@ gimp_image_merge_down (GimpImage      *image,
       return NULL;
     }
 
+  if (! gimp_item_get_visible (GIMP_ITEM (current_layer)))
+    {
+      g_set_error_literal (error, GIMP_ERROR, GIMP_FAILED,
+                           _("Cannot merge down an invisible layer."));
+      return NULL;
+    }
+
   for (list = gimp_item_get_container_iter (GIMP_ITEM (current_layer));
        list;
        list = g_list_next (list))