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.

(cherry picked from commit cc47597a)

app/core/gimpimage-merge.c

@@ -236,6 +236,13 @@ 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))