[gegl] tile-handler-empty: add a memory barrier before cloning the common tile

[N/A <ell src gnome org>]

commit d5a00968c98f86b360ceed5c71e082c5ddc17983
Author: Ell <ell_se yahoo com>
Date:   Fri Jul 21 03:59:04 2017 -0400

    tile-handler-empty: add a memory barrier before cloning the common tile
    
    ... to be on the safe side.

 gegl/buffer/gegl-tile-handler-empty.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/gegl/buffer/gegl-tile-handler-empty.c b/gegl/buffer/gegl-tile-handler-empty.c
index d6dffbd..8bec7f6 100644
--- a/gegl/buffer/gegl-tile-handler-empty.c
+++ b/gegl/buffer/gegl-tile-handler-empty.c
@@ -60,7 +60,8 @@ _new_empty_tile (const gint tile_size)
     }
   else
     {
-      if (!common_tile && g_once_init_enter (&common_tile))
+      if (!g_atomic_pointer_get (&common_tile) &&
+          g_once_init_enter (&common_tile))
         {
           GeglTile *allocated_tile = gegl_tile_new_bare ();
           guchar *allocated_buffer = gegl_malloc (common_empty_size);